You need to have a good background in MUSIC THEORY. and you need to at least be able to play a piano at an advanced level.. For about 11 years now I have been writing music beds for commercials and songs for TV shows and I write on average 20 pieces of music a month.. each 60 seconds long. To get to this level, I have a 55 year background as a professional musician, I have a Master's degree in music and have played all over the world, including 3 world tours. .. but, if you plan to write music for commercials then you will need an EQUITY CARD as a minimum and you will need a LOT of connections because that work is hard to get and you will be competing in a small market with people like me.