Both have a lot of good info on XP, but tigerseye is much more easier to follow.
Go to tigerseye:
Under Basic Tutorials / Events / Tutorial III: Variables. That should cover what you need to know.
In short, a variable works similar to switches, but with more possibilities. You know how you set up a switch and then activate them on/off from any number of events? Well, you can set up a variable, give it a name, and value. Example, new variable is called Apples, and the event for that variable will trigger when you find 3 apples. Then set up your apples events, and for each apple you get, you'll set a command to add +1 to the Apples Variable. Once you get 3 apples, you've met the criteria to trigger the event.
Variables can be a bit confusing to work with, but they become easier after awhile, and makes eventing much easier.