(from someone who has actually done this): My best advice is to first start READING. Two books to read that will help to answer some of your questions (and give you information as to what happens once you get noticed by a major)- This business of Music and All you need to know about the music business (but were afraid to ask)...these texts will tell you the truth behind the industry, and also will give you some insight into things (like how much do you REALLY make when you go platinum-hint: Much less than you think).
Also, realize that there are about 800,000 people attempting to do what you are doing...how would you make yourself stand out from the pack? Having a clue about marketing yourself makes a label more interested, because if they can sell you-they can sell the record...people liked Nelly because he's marketable and a bit different, not just 'talented.'
And...make sure you are thinking ahead musically...What's hot today may not stand up in 6 months to a year (or more if you get a deal with a major)...so consider working towards something musically and lyrically that will stand up and not just be a hit today, but have staying power-this type of thing gets labels hot for you.
Oh...and a couple of extras-
Never hand out a demo without email and contact info.
Never give your music to anyone who says they are a 'producer' or they "know" (insert name of famous rapper here)...chances are they aren't or they don't...and they want to scope you out...for this...I suggest the following:
Get a myspace page and put your demo up there. Make a business card with the address of your page on it...and anyone who wants to hear it, can. For free.
I hope this helps you, I wish you good luck-being a professional for over half my life (19 years) has not been easy at all, and sometimes you find out that doing what you like is better than making it huge.