DNA will not solve all your genealogical research problems and your DNA test results will not spit out every precise relationship you have with other testers.
It can confirm close relationships relatively easily–parent/child or sibling, etc. Certain types of DNA testing can confirm more distant connections along certain biological lines (mitochondrial for direct maternal and Y-DNA for direct father-to-son, but neither will “prove” exactly how many generations back the connection is) and autosomal DNA will indicate a relationship exists between two individuals but does not specify the type of relationship or where the “fit” is in each person’s pedigree. It takes work to figure that out.
And DNA testing cannot be used effectively without paper records–particularly autosomal testing.
AncestryDNA is $59 through 27 November–a really good price. But remember the pitfalls before you test