[FwL]

There are (expensive) books you can buy on style and form, but you can just get started doing it. A program like Finale already takes care of the form issues in a rough way. If you were going to submit to a publisher, you would want to clean it up quite a bit, though.

The biggest thing you'll face is making decisions about how to present something. A good rule of thumb to follow is the easier to read and interpret the better. The one mistake that I see almost every inexperienced person make is obscuring the beat in a measure. A simple thing like not writing note values across the mid point of a measure can make a huge difference in readability.

Like anything else, you can start simple and work your way up to the more complex. Copying existing music into the program is also a good way to learn the program and get a feel for how things are notated.