Aww, browsing the other character boards late at night brought me a smile
I can't give Luigi advice, but I can give advice on getting into the game in general.
1. Play the game by yourself first. This means going through Classic mode with several of the characters (if not all of them just to see what they're like). At least try to unlock all of the characters first. This will get you acquainted with how the game works (though don't get used to items lol, they won't matter for long).
2. Play the game as your chosen main. If you do choose Luigi, then play as him against CPUs so you get more acquainted with his moves.
3a. Come read a guide. The Luigi Boards have all of their important threads listed in one thread, and you can find a slew of information to take your game to the next level.
3b. Watch videos. See how the pros do it, remember what they do, and try to emulate them. Don't just watch videos; pick them apart and analyze everything.
4. Play other people/wifi. This can be brought in earlier, but it is especially important after all of the other steps. After learning what your character can do, you need to play other people to learn how he/she actually does in matches. This will be the longest stage of development, as it's an ongoing process and takes a while to even get up to speed.
5. Don't get discouraged. Competitive Brawl is not an easy thing to get good at. It may seem lighthearted, and I supposed compared to other tournament games it is, but there's still a lot of work involved in progressing, and you're going to be bad for a while. You say you're a Halo guy, so I doubt you have a problem with fighting through the noob stage; just stick with it.
6. Don't main Metaknight. Ok, maybe this one isn't a rule, but if you don't know already, he's accepted to be the best character in the game. If you choose him, prepare to have your character constantly hated on (cuz haters gon hate).
And that's about it. Brawl really is a fun game to play; the wide cast of characters and stages provides a lot of variety in gameplay, and since every character has a unique personality you feel closer to him/her than you would just a guy in a suit with a gun. Also, the communities great and (mostly) friendly, so welcome to Brawl and enjoy it.