How to Compose music

Hi I am thinking about composing music but there are a few things that I need to know. First can you compeletly compose music on your computer with the instuments sounding like the instruments in real life? Second when you compose music with multiple instruments some of which play at the same time how do you do that, I mean do you have layers that you put each instrument sound on?

You need a sequencer or notation software and orchestral samples. Either case provides tracks or channels to which you assign your instruments/samples. Depending on the setup you choose you may have to allocate different articulations of the same instrument to different tracks, like violins pizz, violins arco loud, violins soft, and on. Unless you're writing music at a standard mezzo-forte, no samples deceive the critical listener but you get some idea of what your piece should sound like. To make sampled stuff sound more convincing you need to know a fair bit about the instruments themselves. You only need to worry about this if you want to imitate a traditional orchestra or band.

You can compose straight into a computer simply using trial and error until you get it right; or hearing it in your head and simply programming the sequencer or notation...depends how you work.

It's worth learning about form and harmony to the extent that you can control what you're doing. If you have the talent naturally, fine; if not your music will probably sound untutored or ill-considered. So you need to develop a critical ear so you can move forward and improve as you gain experience.

So when you are composing music that you are going to publish do you use real instruments and record the music? Also I have heard that most insturments take a long time to learn so how come some composers can compose music with several diffrent instruments when they have only been composing for a while?

By publish, do you mean creating sheet music or do you mean creating a sound recording of the music?

To answer your question about how composers compose music for many different intstruments, the answer is that you must learn about them. You don't have to be a master at them, but learning about them and how they work is crucial to creating a functional composition for them. Any instrumentation or orchestration book will get you started in the right direction. Thankfully, one of these resources has been posted online here:

Principles of Orchestration On-line - Northern Sound Source

As to recording music, many people use synthized or sampled instruments to make either mock recordings to get an idea of what the piece will sound like with a full orchestra or, in the case of most television shows, video games, and the like, use these recordings for the final product. There are many tools out there that will help you achieve that, but I suggest before making a humongous purchase that you really decide what you will be doing with these recordings and if you really need them.