The main place you can read up on the system is at the discussion forum here
; I'd say playing without having intimate knowledge of the rules is fairly possible, as long as you're willing to follow the game's philosophy of "Say what you want to do, then let the GM figure it out." (since the game's rules generally translate narritive into mechanics, rather than the other way around)
At its core, the game is a dice pool system, where your dice pool is built from basic components:
Your affiliation die (a die which you always add, based on whether your character is better solo, or in a team)
1-2 distinction dice (dice added based on your character's personality, and the environment at hand, such as if you're fighting atop a skyscraper)
A power die from each power set (dice that represent your superpowers)
A specialty die (dice the represent your character's skills)
You can also get a few more dice from things like your opponent being hurt, setting up assets to help you, and bringing things like a gun to a knife fight, or a computer virus to hack into a security system, but the basics are that everything in the system is rated from d4 to d12, representing the size of dice you roll.
That should be an introduction, and obviously not enough information to get going on, but I have experience running the system, and the basics are that it is self-balancing, and allows Hawkeye to operate right next to Thor without much trouble. (which is why there's no character creation restrictions, aside from the judgement of the GM whether certain themes or characters are appropriate.) In play, the system offers a lot of freedom, as long as the players follow a strong narritive.
Here's the example of play, to see how everything fits together: http://www.margaretweis.com/images/stories/bonus_content/mhr_exampleofplay.pdf
Captian America's Character Sheet: http://www.margaretweis.com/images/stories/bonus_content/data_file_captain_america_promo_2.pdf
Random datafile generator, containing advice on creating a character: http://www.margaretweis.com/images/stories/bonus_content/mhr_randomdatafiles.pdf
(Note: Do not rely on the random generator to create your character, since the character will be too weak for this series.)
Standard list of superpowers: http://boldpueblo.com/downloads/mhr/MHR-Standard-Power-Traits.pdf
Standard list of special effects and limits: http://boldpueblo.com/downloads/mhr/MHR-Common-SFX-and-Limits.pdf
List of fan-made character sheets: Datafile Index
Game reference cheat-sheet: http://nerdnyc.com/tmp/marvel/Hero-Cheat-Sheet.pdf
Collectively, that should be enough to create a character from, though in Marvel Heroic Roleplay, it's much better to start from a concept and work towards stats, than the other way around.