Buy a thesaurus. This will help you build your vocabulary and find rhyming words that you'll need to rap creatively. A dictionary will help as well.
Study song writers you are inspired by, and study rappers you want to be nothing like. Both are important as you build your own style. Learn what you love about some writers and what you hate about others.
Practice freestyle rapping to a beat so you can learn timing and thinking on your feet. Use random words and try to rhyme them, but don't sweat the content. The important thing is to stay with the beat of the music and keep your words flowing.
Practice speed and tricky turns of phrase with lyrics from other writers. This will help you learn flow and delivery. Once you have that down, start adding your own words.
Carry a notebook with you. You never know when an idea or inspiration will hit you. Be ready to record it when it does. Make lists of rhyming words in the back of the book. Rhyme single words as well as groups of words.