If you are looking for DNA matches it is best to try in various places. The big one is GEDMatch, that is where many people add their DNA kit to in order to find matches with people who have tested at the various DNA testing companies.
Family Tree DNA is my favorite DNA testing company because they offer three different DNA tests and seem to have a good interface to looking at your results and matches.
Ancestry DNA test is all right, you don't get as much detail on the matches as at Family Tree or GEDMatch but they have a large database of Ancestry users who have taken the test. And their DNA Circles feature automatically groups together uses who share a common ancestor.
One site I do enjoy is MyHeritage. You can make a free account and then upload your DNA info to the site. They seem to have a lot of people taking DNA tests with them and they have a nice interface to compare your DNA with your matches and also find out who else shares DNA with the match you are looking at.
There is also Geni, where you can add your DNA to your free account, it will give you matches too and if your family tree on Geni extends out enough it will tell you how you are connected on the paper family tree too.
And of course here on WikiTree, add your DNA info and it will propagate out to your ancestors and help other DNA matches find you!
Oh, there are also "Projects" you can join at Family Tree DNA such as the Caribbean Y-DNA Project.
Then there is the DNA Painter website for when you get really pro, where you can map out your specific DNA segments and which ancestor they trace back to!
Overall DNA matches can help you piece together your family tree, but really you will need to work on your "paper tree" as much as you can too in order to make sense of the matches you find and to be able to place them in your family tree.
I find that Genealogy is more of a lifetime project then something you take one DNA test with and figure out everything at once.