Once you have the tortillas sussed, burritos are easy.
You can really put just about anything in them. I usaually put in cheese, beans (either whole or refried), chopped cooked meat, chopped bell peppers and corn. Ohter good fillings include sour cream, and leftover rice. I tend to put the salsa on after I have cooked them, but some people put that inside too. They are a great way to use up leftover roast meat (inc. chicken).
Put the filling in a line just to the right of the center line of the tortilla. Fold the right side over the filling, then fold in the top and bottom end, then flip the filled bit over onto the rest of the tortilla. I usually heat them in the microwave, though a more "authentic" way would be to start with warm fillings and pop it on a griddle for a minute. YUM!