How to Rap (from wikihow.com)

Steps
-Listen to the beats of the instrumental.
-Bop your head and get a feel for the rhythm. Just groove.
-Start saying words in rhythm and try to rhyme. Work on your flow -- this means that each line should have about the same amount of syllables.
-Start reading a dictionary to improve your vocabulary, and have a more vast reference to draw from.
-Listen to different types of music, for this will expand your creativity.
-Perform at local shows if you can. When you do, look at the reaction from the crowd or have it videotaped to see how you can improve.
-Stress the consonants. If you try to rap the way you talk, it won't be comprehendable.
-Write everyday
-Freestyle everyday (alone or with others)
-Practice by just reading the lyrics. Then, read the lyrics like you're trying to get a raise from your boss. Try to get it on the beat. Imagine when you are rapping that you are doing it in front of someone who knows you well and you respect. Don't try to make your voice sound like someone you're not. Relax.

Tips
-Download rap instrumentals like you would download regular songs.
-Try rapping to slow-flowing songs before trying to rap quick--you are not Krayzie Bone!
-Don't think a word can only rhyme if it's spelled the same. It's all about your flow and delivery.
-Get started improving your vocabulary.
-Use metaphors. Here's an example: "I`m a player in the game but I don`t follow the rules man/I`ll have you posted on the wall like a nerd at a school dance"
Write at least 10 rhymes a day. Even if you don't like what you've written, later on you could come back and re-word those rhymes into something you do like. When you finally like what you're hearing, test them in front of your friends and see what they think.

Warnings
-Be careful with racial, sexist, or hate lyrics or anything like that, it can get you in trouble.
-Don't steal other rappers' styles or lyrics, but learn from them and incorporate their style into yours if you want.
-Don't try to sound like you're someone you're not! If your voice is not low like Biggy then just accept it and work YOUR voice into your style, don't try to pretend.
-Don't rap to a beat that you're not comfortable with. That means: Don't clump too many words to a sentence to fit the beats, especially if you are doing a parody if you will just end up running out of breath or have really watery mouth. And don't rap a song that is too fast for you to begin with. Learning to rap is very unlike singing. You can't rap-along to songs on the radio. You will hear Fantasia singing 'Sorry Seems To Be The Hardest Word' by Elton John, but it is highly unlikely that you will hear The Game rapping to Eminem's 'When I'm Gone'.
-The reason why people will never pick a rap song to sing in a karaoke is this: While you can take advantage of the 'transpose' button to drop or raise the key to suit your own range (which is how most cover songs are done), there are NO keys in the system to let you speed up or slow down the tempo of the song, unless if you have a live band or something.