Pls Explain Online MP For Noob

I finished the Campaign Missions for H1, H2, H3,H4, ODST and Reach. I want to learn how to play online multiplayer but know nothing about it.

Are there just a few Maps that people play or there a lot?
Is there a way to practice the maps against the computer before playing against people?
Does matchmaking really work? Will it put someone like me who has never played MP with similar people or not?
Any tips or info would be greatly appreciated.
Thanks.