the type of software you want depends on what you want it to do. is it for recording your compositions? scoring them out?
also what kind of compositions are they? for example if its a drum and bass piece you might like to try out Acid (no, not the drug) combined with soundforge....but that would be much less use if they're classical.....
music software is a complicated area with alot of diferent choices, so maybe you could do a few google searches and see if theres anything there that can help you narrow down what you're lookin for. the 'sound on sound' website (its a magazine) has a lot of good articles and might be able to point you in the right direction.
hope ive helped
oddy