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 If it is outdated, on a website with open editing, or not written by an expert, it is always a good idea to find another source.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!