To add to what ClaireB has written –
Choice of testing company and the type of test depend on what you want to achieve. Do not expect too much. Arguably the best two for genealogy are FTDNA and Ancestry. I have taken tests with both.
Neither FamilyTree’s 37-marker Y-DNA test (applicable to males only) nor the autosomal test (‘general’) with Ancestry produced much information and the little bit it did has not been of use.
My Y-DNA tells me that my “Predicted Haplogroup is R-M269” and probably Z253. Great, but M269 is the dominant lineage in all of Western Europe. My 37-marker test produced 2 matches at a genetic distance of 2 and 15 matches at 4. None of them bears either my surname or a name in my (almost 700 member) family tree. Because I have no close matches it is now suggested that I do a 67 marker one to refine the results.
My Ancestry test produced one ‘find’ – a first cousin once removed that I knew about. It shows 2 people described as third cousins i.e. we share one of our 16 gggrandparents (statistically I have about 200 third cousins). Neither of them have surnames I recognise, neither has logged on to the Ancestry database in months, nor have they responded to emails. Also neither of them have uploaded trees, same as most of the other matches.
It is interesting to look at and learn about the process, but genealogically DNA testing has done nothing for me.