You don't have to be 100% familiar with any game's rules to GM. The main attribute (and it's required, no matter the rule set picked), is ability to guide the players through encounters and adventures.
It's recommended that you give a brief look over the current and former games in the MRG to see how different GMs have guided their players through various games.
As for rule sets, there are a variety of options. The simplest is "power of plot". There are no rules and the plot takes precedent in determining the outcome of any encounter (some players don't like it because it takes randomness out of the game).
Other options with the basic rules online* (so they are easy to reference) are:
Palladium:
http://www.macrossroleplay.org/Sketchley/Statistics/Rules/MacrossMicrolitePlatinum.htmMekton:
http://www.mektonzeta.com/MektonAlpha.pdfMini Six:
http://antipaladingames.com/Mini_Six.pdfThough, it must be stressed that game mechanics only really come into play during combat and character creation. Even then, a lot of us GMs don't have the time to roll out all the dice to resolve combat all the time, so part of the time, we're GMing by the "power of plot" with the dice only serving to randomize things.
Nevertheless, check out the games in the MRG. And keep in mind that GMing only requires a small set block of time, one day a week (it can be as short as 15 minutes, or considerably longer, depending on how fast you type, how quickly you come up with ideas, and how much descriptive text you add). It's not hard.
* That I'm aware of.
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