Try and tailor it to the type of course you are applying for. If you're applying for music at a traditional university, the course will probably involve more history of music, composition, aural, score analysis etc. than places such as RSM, RAM, RNCM, Trinity, Guildhall, or Birmingham Conservatoire, which focus more on perfoming. Try and cover all the aspects of the course in your statement, and talk about why each one interests you.
Don't go on and on about performing. Everyone applying for a music degree can perform, and if you talk about it too much, they might think they are your backup option in case you don't get into a music college. (Even if that's the case, you don't want them to know that!)
If you're interested in composing, talk briefly about your favourite composer and what you like about their work. Mention wider reading you have done, genres of music you are interested in, and what you feel you would like to study in greater depth. If you know what you want to do with your music degree, mention that.
Above all, you need to convey that you are absolutely passionate about it. It may seem easier to talk passionately about music than a subject such as physics or maths (no offence to physicists or mathematicians!) but you don't want to waffle on about how lovely it is. Be specific. Talk about areas which really inspire you, and what you look forward to doing the most.
I'm not applying for music, but I know a lot of people who are/have and I'm in a music tutor group at college, so if you need to know any more, please ask and I will try and find out for you.