Listen to a lot of music and familiarize yourself with music theory. In order to properly produce music, you'll need a sound understanding of how instruments and sounds interact with one another. Listen to oldies and new hits to gain as much perspective as possible on all types of production techniques.
Learn how to play as many instruments as possible. This will help you understand the mechanics of the instruments and how to make adjustments to achieve the best sound possible. You will also be better able to explain processes to the artists you are producing and give examples of what to play, if needed.
Learn from professionals. Attend a music production school, if possible. Get first-hand experience by sitting in on recording sessions and watching what successful music producers do. Learn how to use a mixing console and other studio equipment.
Familiarize yourself with modern production technology and technique. For example, modern producers should be aware of digital recording technology, sampling and beatmaking.
Offer sound advice on songwriting, arrangements, choice of instrumentation, etc. For example, if the verse in an artist's song needs tweaking, ask if he has an alternate verse or offer your own ideas.
Coach the artists you are recording and encourage a productive and positive working environment. For example, if a vocalist is having trouble hitting a certain note in the song, offer words of encouragement and give advice on how to best reach the note.