There are many wonderful guides for getting started in genetic genealogy.
This is not one of them.
This checklist is for those who are serious about using genetic genealogy to bust down brick walls, who plan to avail themselves of all of those proper tutorials, and who just want to sort out their mise en place before things get rocking.
Are both of your parents alive and willing to test? Then test them, not you. They have twice as much ancestral DNA to explore.
Any grandparents alive and willing? Great-grandparents? Siblings of any of these? Run, run and beseech!
If you have to make decisions based on budget like the rest of us mortals, then begin with whoever is most likely to, ahem, not be able available for testing in the future.
Keep in mind that even if you test only one person, taking multiple tests is ideal. That said, it's better to have tested many relatives one way than to have tested one person several ways.
Be mindful that 23&Me's test also provides health-related information. Make sure your test-taker is fine with this. My husband wasn't, and so we fish in other pools. If I, greedy genealogist that I am, can handle this, so can you.
FTDNA is the only mainstream company doing "thorough" yDNA/mtDNA testing. They also store your DNA, so you can order one test now and others later without getting a new sample. If you think that you will want to know more about the test-taker's direct paternal or direct maternal lines, also test with FTDNA.
MyHeritage is, some days, my favourite testing company of the lot... but they're, in fact, so wonderful that you can enjoy all of their features without testing with them. (See "Upload" below.)
Not an American? Consider testing here. LivingDNA takes uploaded results from other companies (see next section), but the wait will be an unknown number of months, and you may or may not get all of the features you want (reports vary). American? Only if your budget is Bill Gates-ian and your patience is short.
Sales happen at all or most companies around DNA Day (April), Mother's Day (May), Father's Day (June in the US, September in Oz), and "Black Friday"/Christmas. Some companies, like Ancestry, discount their kits at other times, too, like St Patrick's Day. Google around or hit the various Facebook groups before ordering and see if a sale is likely to come up soon. (The $20 you save is $20 that you can put on another test...)
Don't withhold your family tree because you think playing coy will lead to more personal communication. I almost never contact matches without trees. I do reach out to people with trees when I have a discovery they might enjoy. I just don't have time to woo people who may not even care. Almost everyone has too many other more forthcoming matches to chase up.
The presence of a tree signals that you are interested in family history (and not just ethnicity charts) and shows others what you already know.
If you have the time, work on adding as many cousins as possible using census and vital records. I have been able to quickly sort out literally hundreds if not thousands of matches with no obvious connection because I remember (or can quickly double-check) those collateral line names.
Attach a tree at every place where you've tested or where you've transferred results. You might rely on a link to another site at, say, FTDNA, but at least list your surnames so that the tools will work as well as possible.
Download your raw data from wherever your results come in first and upload it to the following sites. Do at least GEDmatch, even if you tested everywhere, because it's the only way to see segment locations for Ancestry matches.
GEDmatch.com: This is the only site that takes results from everyone. The tools here are powerful and ever-evolving. If you're an Ancestry test-taker, be sure to use a name that makes it easy to find you on the Ancestry site (and attach a tree for the same reason).
The site is free, although at some point you may wish to try the more advanced tools. (Or be like me, pay the $10/month as a one-off for the odd month here and there, when I know I'm going to be doing lots of sleuthing.)
Please consider opting-in to allow law enforcement to see whether DNA samples that they have uploaded of victims and offenders match you. (They don't see your DNA; they just see if you're a match, like any other user can.) If you match, your family tree may assist in solving a crime. If this makes you uncomfortable, then don't opt in.
MyHeritage.com: pretty interface, international customer base, matching segment locations, easy-peasy triangulation identification, and a terrific "theories" tool that tries to predict how you're related to someone.
MyHeritage feels like the reason that other companies finally started upping their game. If we ever get segment inform
FamilyTreeDNA. Be aware that uploaded DNA tests can't be used for yDNA and mtDNA testing.
LivingDNA.com: Uploads are free (as of this writing). The company is reputable. Can't hurt... although in my (American/Australian) experience, at this point it's just a tiny sample of the same lot of matches that you see everywhere else.
Once your results are in, you're going to need a way to track that this chunk on chromosome 3 seems to come from your maternal grandfather, and this other chunk on chromosome 17 comes from your paternal grandmother's paternal grandmother, and oh... it sounds so twisty already, doesn't it? It's not, not if you use some wonderful tools like those below.
I would consider donating a toe to make sure this program doesn't go anywhere, ever. I don't know who is taking toe donations, or why, but GMP is just that good.
These days many people are using DNA Painter (see below) to track all of their matches, and I do treasure certain features at DNA Painter, but GMP will really let you massage your DNA data into complete "Oh yeah, that's the spot, work out those Smith knots..." bliss.
GenomeMate Pro is available at GetGMP.com. It's free as a labour of love from genealogist and developer (and my distant cousin - toot toot!) Becky Walker, but donations help to keep it a top-of-the-line application. Check out the user guide from the "Downloads" section; it's comprehensive and frequently updated as sites/tools change, and support from the Facebook group's community is always fast and friendly.
People use DNA Painter for all kinds of wonderful things, and I highly recommend joining the site's Facebook group and/or the user group, if you want to know more about that. Developer Johnny Perl is a switched-on fellow who is always coming up with a cool new feature for the site.
I use DNA Painter to make segment maps that I can share with the public. (Like on this site!) Should everyone be making public segment maps? Well, it would certainly be useful to see a match, go to their public segment map, and see right away if they've narrowed down the origin of our segment to a particular branch of their family.
(Hopefully that match would also give enough info to say where that theory comes from, which DNA Painter makes it easy to do in a few different ways. Not everyone fully realises that even if someone is really, really, really obviously your fifth cousin and you don't know of any other connection... that doesn't meant that there isn't another connection. The other link could even be more distant and still be the source of the DNA.)
So, map early and often, once you start sussing your results. Link your maps somewhere where matches can see them. Share them with your family so they realise that those hours in front of the computer aren't being spent admiring your "I'm 92% European!" pie chart.
You can also use DNA Painter as your main segment manager if GenomeMate Pro overwhelms you.... but be careful. If you suspect that you're going to really embrace using DNA for family history research, you are far, far better off in the long run if you take a deep breath and give GMP a second chance, ask for help, etc. GMP is just so robust, and Future You who knows so much more will be ecstatic that you did things "right" from the start.
DNA Painter is free for one profile/map. Otherwise, subscriptions are $30 for six months or $55 for a year. However, if you let your subscription lapse, you still keep all of your maps, so I tend to only re-subscribe when I want to try a members-only feature or just to show support.
I love spreadsheets — I will use almost any excuse to make one — but I'd cry if I had to go back to using them for genetic genealogy. (And please note how I put that in bold. I'm finally very serious here.) There's a learning curve with the above tools, especially GMP, but it's really worth it, even if you don't use all of the features.