First talk about which aspects of studying the subject at uni/a conservatoire particularly appeal to you. They want to know why they taking you would be a good move.
Then you can talk about aspects of your subject that really enthuse you: stuff you've been looking at recently, any independent study you've done....
Then go on to talk about hobbies/activities and things... we were told it's best if you can explain how these hobbies and activities have proved beneficial (for example, doing the Duke of Edinburgh Award has improved your ability to commit yourself to difficult tasks.....)
Don't forget any relevant work experience etc. that might count - say how it helped...
Basically just make it clear why you'd be a good student for studying the subject at uni (and that you're a nice person too!)
Don't make it pompous or cheesy... It's really tough writing to promote yourself (especially in so few words) but don't be too modest! Don't lie, obviously, and don't misspell any of it... (someone not going for Music) once said that they were Grade 6-8 at the cello, and got interviewed by an enthusastic cellist...
This probably applies more to general unis rather than conservatoires...Hope it's useful.
2 yrs ago, about 300 people all wrote that their first knew that chemistry was the subject for them when they burnt a hole in their pyjamas with a chemistry set that they received for their seventh birthday.
I'm not joking!
I wrote mine for History, and was told it was a very strong PS... (got offers from Durham, Warwick, York and Birmingham). If anyone wants to look at it as an example, PM me

Mine took 7 drafts - most don't take that long... but just keep at it and you'll find something that suits you eventually. Good luck!