The tutorials here are great for learning how to make the most of Garageband.
Once you figure out how GB works and what can be done with it, with a little work you can create drum loops and modify existing ones quite easily.
You might wish to step up to Logic Express or Pro... both of which are much more flexible than GB and come with many more sounds, loops and instruments and editors to make it easier to create loops... but there is a steeper learning curve for Logic.
There are plenty of amazing Logic tuts here taking you through all the different synths, drum machines, and much more.
Having said that GB is perfectly capable and can be used to create drum, percussive and melodic loops.
Of course you can post any questions you need help with here too!