What tools can you use to understand words you do not know? how can you tell what sources are good and what sources are not?

To understand words you don't know, you can always use a dictionary or a credible website to help. If that definition doesn't make sense, ask a teacher, parent, or peer to help clear it up for you.
Finding what sources are good and what sources aren't, you have to think of several things. If it is outdated, on a website with open editing, or not written by an expert, it is always a good idea to find another source. 
Hope this helps!