There are two ways to cook meringue, I think. Either quite quickly at a higher heat, to crisp and brown the crust, but leave a fluffy inside, or long cooking at a very low temperature to dry the meringue out completely, to make the sort of thing you fill with cream and fruit to make pavlova.
If you're putting meringue on the top of something, you probably want the former method, like on a lemon meringue pie.
Make sure your egg whites aren't contaminated at all with yolk. Make sure there's plenty of air in the mix by beating the egg whites well, and then beating the sugar in well. It should be a very stuff mix - you should be able to hold the bowl upside down over your head (or someone else's head if you're not confident) and the mixture not drop out. Obviously, that's a test you might not want to do until you're really sure...
If you're putting meringue on something (like a layer of fruit in a pastry case or like a lemon meringue pie), the bottom part really needs to be pretty much cooked already, so that the meringue only needs to be in for long enough to crisp and brown the surface.
Good luck!
