How do you know if a DNA company you found online is reputable? And whether its results are reliable? These are common questions. And they are difficult to answer.
If you're asking about tests that will give you ancestry or genealogy information, the International Society of Genetic Genealogy is a great place to turn. Their beginner's guide section has a list of articles to get you headed down the right track. I've also written about choosing between the options in this area of DTC testing before and linked to other articles with advice in this blog post.