People who are new to DNA testing can be overwhelmed by the medical and biological mumbo-jumbo. It is hard to understand what you're paying for, how it works, and what you'll really find out.
A very nice guide, easy to understand, is Richard Hill's DNA Testing Guide, "How to Identify Ancestors and Confirm Relationships through DNA Testing."
And, when you get your test results, don't forget to add your most distant ancestor on that line to the right DNA project on Geni.
That IOGG has some really good information for new-comers as well, including comparisons of different DNA testing companies.
If you have more general questions about DNA testing for genealogy, please read and post in this thread: http://www.geni.com/discussions/80820