Probably the most important thing to remember about family trees is that they should be a starting point for your research and not an ending point. The reason I say this is that in my own research, I have found plenty of misleading and incorrect family trees. To assess the quality of a family tree, look at the source records used to support the entries in the tree. Unfortunately, there a many family trees online that have no source records at all.
Places to search for family trees:
It can become cumbersome to keep entering the same family information over and over at the above sites. I have another free site that simplifies searching for family trees. It allows you to enter an ancestor's information just one time to search for family trees at all of the above sites. Better yet, the site also figures out what would be the best type of family tree search for you for each of the above sites (all family tree sites are not created equal). Go to Family Tree Searcher to see if it makes your family tree search simpler.
If you are not sure that searching for a family tree is the next best option for your research, consider using the advice feature of this site. This will help you pick your next best steps in your research. Go to Genealogy Search Advice.