I work in the legal system, and am an LEO by statute, which puts me in position of rubbing elbows with politicians almost daily. My advice:
1. Go to law school. Politicians are generally in a position of either writring or enforcing laws. A law school education will help should you get in one of those positions. It will also help with the public speaking aspect of the job.
2. Practice law for a while. Try a job as a prosecutor or defense attorney. This will give you contact with the real people that you would be serving, and in a situation where you actually have to learn to talk with them, to make yourself understood, and also to understand them.
3. Develop a thick skin, and control your anger. Politicians get abused. Your ability to deal with it will be essential. Learn to listen before speaking, and answering rather than reacting.
4. Become a good sport and a poor loser all at the same time. You should never like losing, but you also need to be a good sport about it.
5. Money, or connections to money are essential, because campaigns cost money, and much of it will come from your pockets.
6. Keep your word. Don't promise what you can't deliver.