how do I use garage band for beats?

well, your question is a little open ended so I’m not sure what you are asking. Please eloborate.

there are several tutorials online at Apple’s website if you go to the iLife section. Also if you have the latest version there are some included tutorials from the main startup screen. Lastly if you really want to learn the ins and outs of the program I would recommend looking up the text by Peachpit Press “Apple Pro Training Series: GarageBand.”