Generally MASI events are open to anyone 18+ who can follow the MASI code of conduct and the specific rules of any game they participate in. Younger players can attend games depending on that game's policies - they may need to be accompanied by an adult.
Anyone attending an overnight event should be aware that the MASI Site is a wilderness campground - you will need your own tent, sleeping gear, food, sensible shoes, bug spray, sunscreen, and water at a minimum. Some players choose to return home during the overnight period.
You do not however need to worry about costumes, foam swords, spell packets or the like as MASI is happy to loan such items to new members. If you want to make your own gear, some help can be found HERE .
You should be at least somewhat familiar with the rules of the game you will be attending, although you do not need to memorize them by any means.
A good way to start is to check out the MASI Core rules used (with small variations) by all games. These are found in the first chapter or two of each game's rulebook and are mostly a list of safety rules and game effects.
If you are going to NPC, all that is needed is a knowledge of the Core Rules!
Specific Game rules, generally covering character creation, can be obtained by contacting a game master.
Everyone needs to sign a waiver which is available HERE, those under 18 must have it signed by a parent in advance, those 18+ can sign their own at check in.
It is a good idea to arrive early to your first event and let the staff know you are attending so they can show you around as the site is quite large.