They've been plugged on this blog before, but I'm going to plug them again: subject research guides.
Maybe you're supposed to find some journal articles and want to use the library's databases, but you don't have a clue which one to start with. Maybe you've tried Academic Search Complete (a great general database to start with, by the way), but it doesn't seem to have what you need for your newest assignment.
The library's A to Z database list can be a bit daunting if you're only familiar with a few of the databases (or none of them!). The library's subject research guides can help. Choose whatever subject in the list best matches the topic you're researching or the course you're doing research for, and you'll be brought to a page featuring (among other things) a list of the best databases to use. You'll still have to choose from a few different ones, but it's a much more focused choice.