Steps
Create a drum beat. The drum beat must be consistent throughout the whole song and effortless for the listener to follow. You always have to include a Kick and a beat, meaning it should not go out of time.
Create a bass line. The bass line has to be played by an instrument with a very low tone (Bass Synth, etc). The bass line should be in the same key as the melody so that there are no 'sour notes' and the beat is easy to listen to. The bass line should not be as lengthened as the melody line but should work with it and fit together.
Structure the song. A popular song structure would be: Intro, verse, chorus, verse, chorus, chorus, middle 8 (AKA breakdown), chorus, outro. The intro usually consists of all the parts playing, the verse would be just the bass and the drum beat, then the chorus should be all parts playing, the middle 8 (breakdown) should be anything of your choice but must distinguish itself from the rest of the song and the outro is simply the beat just playing on until the song ends.
Write lyrics. Rap about anything but remember to write to the beat and make sure that your 'flow' is not whack! Just make sure that the lyrics match or just associate with the name of the song. Put your heart into it, be in love with the song when recording it or writing it. Just keep talking and babbling.
Listen to your inner rapper to figure out what works best. If you are not sure what to say, remember that is the point, getting beyond your mind/memory. Make up sounds and let new language emerge. Try focusing on famous musical artists you respect/love ones/Jah/your heart and see if that affects what comes out. Focus on rhythm and feeling... Let go, flow and appreciate...



Tips
The hook should be something infectious and catchy. Listen to songs such as: "Imma Shine" by YoungBloodz, "Big Pimpin" by Jay-Z, "Drop It Like It's Hot" by Snoop Dogg, "All about U" by 2Pac, "S and M" by PMF, and "Bang Bang" by Dr Dre. The hook is basically the catchy punchline in every chorus. Like most of those songs listed, listen to the way the artist works with the beat to create a hook and do the same with your beat.
Add flavor to the beats by including drum fills (e.g. before chorus or verse, add additional bass and melody lines and make the song shine).
When creating a bass line: the bass could play a note on every kick of the drum beat.
A hi-hat can be good, because it keeps the song flowing and keeps the pace, but it's optional.
If you don't have access to studio equipment, the following cheap packages can help with your quest: Fruity Loops, MTV Music Generator, and Hip Hop Ejay.



Warnings
Don't rap about personal things that are not true. While some people might take a "I can rap about anything I want to!"-attitude, it's best to really look at things before just spitting out false trash. Trying to beef up your image without doing the "work" is almost universally known as posing.
If you've never been to jail, shot someone, done or sold drugs, had sex, etc, you might not want to rap about doing hard-time, killing or maiming people, pushing or tripping out, and all the people you've slept with. This just comes back to bite you in the butt make you look fake, damaging your image and sometimes putting you in danger.
Also keep in mind that just because popular rappers write about that stuff, doesn't make your raps any more or less "rap" by writing or not writing about similar things. Remember that creativity and vision are often valued more by fans than image alone.
Don't listen to experts, listen to your heart...