Ez Big Z,
I'm one of the resident MCs for Breakneck, an event in Portsmouth, and have been MCing for around 5 years now.
My advice:
Enjoy yourself, and be yourself. Write lyrics that apply to you (things you find funny, things that affect you, ditties on popular culture etc.), not lyrics that are created through bullshit like guns, fiddies, hos keepin' theys legs closed that has absolutely no relevance to you whatsoever (N.B. if the aforementioned do have relevance, ur in the wrong forum).
Really get a feel for the music, understand the different types within the genre, and allow your lyrics and what you say or do to correlate with particular tunes etc.
Respect the DJ. This is paramount. Don't feel afraid to shut up when you're MCing even tho a bunch of sweaty ****s hanging over the barriers are trying to make u do an impression of a machine gun. People will notice and respect it.
Form an alliance with a DJ; creating a musical relationship with a DJ works wonders. You will both learn how to work with the other and produce the best sets. Look at Friction and SP, Swift and IC3.
Be clear in what you are saying. Project your voice, don't shout. Sometimes the MIC levels will not be on your side, sometimes they will. Work with them, shit happens.
Slow lyrics: make them trademarks. Make them catchy. Be proud of them. Work on them loads.
Quick lyrics: don't make them too complicated or cram too many words in a line - concentrate on a bouncy rhythm and some cheeky internal rhyming. This often helps. Don't go biddibiddibiddibiddibo.
On lyrics in general: read into how rhymes and rhythms work. Couplets, Triplets, Quatrains etc. Rhyme schemes abab, abba, etc....play around with words. Tetrameters, pentameters. This is sounding really geeky, but I do English at Uni, and a lot of the stuff I learn in poetry i apply to lyrics. Read up on it, they sound so much better if they follow structure.
Learn what works best for you: have a good knowledge of tunes and an understanding of musical patterns (bars, phrases etc). You will find out with time.
One last thing:
Practice.
Hope that helps,
Gibs