Oooh, hello and welcome to the h2g2 Munchkins RPG. This is, essentially, a game for people who are already gamers. That's not to say non gamers won't enjoy playing, but there may be references which make more sense if you actually know what they're referring too.
Setting, classes etc.
The game is set in a sort of generic D&D high fantasy world, much like Greyhawk, Dragonlance or WHFRP*.
When you choose your character you'll have to choose a race, alignment, primary class and secondary class.
That is, what species you are, whether you're good, lawful, evil, or chaotic, what job you have, and what type of personality you have, eg you may be a Half-elven/Lawful/Ranger-Real Roleplayer, or you might be a Human/Chaotic/Black mage-Munchkin...You might equally be a Slime/Lawful/Lawer-Loonie it's entirely up to you, whithin the bounds of types permitted.
500m Sprint...nah, just kidding
Pretty much any race is permitted, ask and, so long as it's deemed viable, it shall be given. The only definite exception to this, at this stage, is Dragons. Daconis Nobilis, Chromatic Dragons and Metalic Dragons are all a big fat no-no.
They are just not seriously viable in a game where classic dungeon crawls will be involved. You could, of course, be a swamp dragon, Draconis Vulgaris, but as shiney surfaces, too much spice in your food, getting excited, and just generally living would be likely to make you explode then this may not be a good choice for your race.
The possible alignments are
Good - yeah, a goody goody who doesn't like other people being even slightly naughty or in fact any naughtyness.
Evil - The exact opposite of good. Loves naughtyness and planning the conquering of the world, or at least part of it.
Lawful - Not the same thing as good. Lawful characters obey and like rules. Rules are everything. Evil characters can be lawful, such as fascists.
Chaotic - Chaotic characters are anarchistic and see no reason why they should follow any rules if they don't want to. They can be loveable rogues or nasty people who want to bring about the end of the world.
Neutral - Neutral characters fall somewhere in the middle of all of these. The may bend the rules a little but will probably follow them most of the time as that's often easier. They may do good or bad things depending on how they feel at the time.
This is your job. You may be a cleric*, a smith of some sort, a White-mage, Red-mage, Black-mage, a mime, a dunnikin diver, it's almost entirely up to you. Again, like with races, if you want to be something unviable, e.g. Spacemarine, then it will be disallowed.
Real-men* - Real-men are the stereotypical fantasy barbarian hero...or American footballer. Big on courage and muscles, short on brain power. Real-men are overly macho and will charge down a Chromatic Dragon with a basic bronze sword. Mainly fighty and brave.
Real Roleplayers - Real role players try to think their way out of trouble as much as fight their way out, not so brave/stupid as Real-men but not too silly or sneaky either. A mixture of Fighty and Thinky.
Loonies - As the name suggests, loonies are not the sanest of characers. They will often come up with insanely convoluted and silly ways of solving a problem, these ways may involve jokes, puns, and references to such things as sci-fi/fantasy books, films and TV programs.
Munchkins - Munchkins only care about two things, loot and staying alive. To that end they will create a character that has some sort of advantage in either or both of those two areas; they may be especially fast so that they can escape easily, or steal from people before the victim notices, or they may be very poweful. Munchkins love magic weapons and armour and will willingly stab a fellow adventurer in the back in order to get the victim's loot and leave a nice distracting wounded adventurer to keep whatever monster was chasing them busy.