Only when you draw that card. As they state in the rules, when you start, you have no class (ha!) and no race. So if you draw a race/class card, you get to play it. If you have more than one (say, in your initial hand), you choose one to play (unless you get a Dual race/Dual class card, in which you can play both (or 3, since you have to play the "Dual Race/class" card as well.
Nope. You fight based on your level, plus the "pluses" of any gear cards you have (like "Butt-Kicking Boots" or "Dragon Flagon"), plus any situation cards you might have. You can also play situation cards on other player's monsters, like "And it's mate!" or "baby of it's kind". Obviously, you want to play the ones that add to monster level on other players (This is the "Screw Your Friends" part of the game), and the ones that make combat easier, on your own monsters.
I also own "Munchkin Fu", which is the game in an oriental, "John Woo movie" sort of vein, which has "Styles" of martial arts, like "Toe Fu" that allows you to wear two sets of footgear at once, and "Pie Fu" (gives a bonus to running away when they are "covered in the butterscotch of your ancestors"
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The guide don't really help me it seems hard to follow, so anything you can tell me can help.
I also own "Munchkin Fu", which is the game in an oriental, "John Woo movie" sort of vein, which has "Styles" of martial arts, like "Toe Fu" that allows you to wear two sets of footgear at once, and "Pie Fu" (gives a bonus to running away when they are "covered in the butterscotch of your ancestors"