The 501 is a two ball trick, not too hard, but not very easy either, because the trick will force you to work against your reflexes as your hands will want to catch the wrong ball. If you don't understand the name, it comes from siteswap, a system of describing some juggling patterns with numbers. Some jugglers love it, others don't.
To start practicing, take two balls in your right hand. Throw a high throw to the left hand, as high as you can, to give you more time. Now, don't catch that ball yet! First you must pass the other ball from right to left and throw it back to the right, as high as the first ball. Then you can catch the first ball in the left hand and immediately pass it back to the right, before you catch the second ball.
It's tricky, but look at the picture above and don't let your hands catch the balls too quickly. Try this starting from both hands, and when you are good enough, move to the next step.
Now, all you have to do is to try to continue in the same way as before without stopping. Throw-catch-pass, throw-catch-pass...
When your hands get the idea, you can go on forever, it's one of those patterns that just 'clicks.' And those are the patterns that teach you the most.