DNA tests come in different varieties that each accomplish something different. It's important to know what you are looking to get out of the DNA test before purchasing it.
Autosomal DNA tests are probably the most accessible and popular tests available now.
Autosomal tests can be taken by either gender and will provide ethnicity percentages as well as tell you how much DNA you have in common with another person.
Get an overview of autosomal DNA testing in this video and article.