Read through a rhyming dictionary or thesaurus to gain an extensive vocabulary. A vocabulary is a freestyle rapper's biggest tools. The more words you know, the more complex your rapping style.
Write down metaphors and similes as they come to you. For instance, "Crooked as Denzel in Training Day". When you compare one thing to another, your raps become interesting and catchy. Similes and metaphors will separate amateurs and professions very quickly
Write a rap to start out with. This could be a two line hook or a whole chorus, but it will be something to fall back on when you stumble with freestyle.
Practice thinking about what you are going to say next while rapping. This is a skill that can only be achieved after much practice. Go to a secluded place and pick a couple of topics to rap about while you exercise this skill.
Read a newspaper and keep up with current events. Putting current events into your raps is an excellent way to gain attention. This makes your rapping contemporary.
Take a public speaking class. This may seem strange at first, but a public speaking class will teach you how to enunciate your words and speak to an audience. It will also teach you how to get over stage fright. Public speaking classes are held at community colleges and universities in all areas.
Take a music class. Music classes will give you the basics of beat and rhythm. Most rap songs will take the 4/4 time signature, and you need to know how to manipulate these rhythms in real time to keep your rapping fresh and individualized.
Enter a rap battle in your area. They can be found at local nightclubs or social events in the community. There may be an entrance fee, but you can get a lot of experience and take notes from other rappers.