I recently had my DNA tested. After a little research, I chose Ancestry.com b/c 1) it’s easy to use; 2) huge number of family trees to match to (plus historical records to verify and document your tree); 3) they keep the DNA, so as technology improves you may get more results; and 4) the raw DNA data is easily downloaded, then uploaded to other sites with more powerful technical analysis tools.
I’ve uploaded my data to Family Tree DNA (for a moderate fee) and to GEDmatch.com (free). These allow you to match one-to-all (a fishing trip) or one-to-one (to zero in on someone who looks like a genuine match). I am barely beginning to understand it all, but am pleased so far.
Also, my family tree is complete (as far as I know it) on Ancestry and is public.
I have found the tutorials on the site linked below to be very helpful, including Lesson 2 which helps you figure out which company to start with. The answer may depend on where your ancestors were from, as well as what kind of information you are seeking. If you are looking for relatives within the time frame of a typical family tree, don’t skip the autosomal (atDNA). The others will take you back into deep time, but won’t show spouses, other children, cousins, etc., just a son-to-father-to-father chain or a daughter-to-mother-to-mother chain.
On the other hand, for African Americans searching for a white male ancestor, the Y-DNA test may be the most efficient way to start.
GEDmatch.com has a whole page of links for learning, under the title DNA for Dummies.