Q: My Mac OS X is running s-l-o-w! What can I to make it faster?

Every time I tell my Mac to do something, he sits there and has to think about it before the actual job gets done. Like a simple request, draw a box and command D to place a photo in, Mac goes into this spinning wheel like it's trying to remember how to do something. How do I found out how much memory I've used? Do I need to take some files off my HD?