If you are going to use a computer to create music you'll need some software too. Apart of your OS you'll need other things:
1st. A sequencer. There're a lot of software sequencers available. May me the most used are Cubase, ProTools, Logic or Sonar. These are commercial software, and most of them are quite expensive, but you can find some free sequencers too.
There is a free version of ProTools (may be the most used proffesional sequencer), but there're other options like Emagic's Logic fun, a free version of Logic, it famous sequencer.
Anyway, most of the sequencers works in the same way and have similar functions: MIDI & Audio tracks, mixing table, plugin support, effects, etc. So surf the web to learn more about these products, check your pocket to see if you have enough money to buy your favourite sequencer or download some of the free ones.
2nd. Samples. If you don't own a keyboard with builtin sounds you'll need a sampler to store the sounds you'll use to create your music.
There are two sample types: loops or 'single note' samples. Loops are usually rhythm samples which can be used to create the rhythm base of a song. They can help you to build a song in few minutes, and they are good to inspire you, so they'll help you to find the melody or song you're looking for. The disadvantade is that they aren't very flexible, so you'll be limited when you use them to build a song.
In the other hand, 'single note' samples are very flexible, due they only contain a single note of a certain instrument.
Using some of these samples you can create a virtual instrument. This virtual instrument is built using one sample for each instrument's note. This allows you to play a sitar with your keyboard, for example. Anyway, you'll need to work a lot in this samples to get a credible real instrument sound.
Usually, the most credible instruments samples are percussions, due these instruments are single noted. But with brass or strings ones you'll need to spend some time until you get good results.
There're tons of samples/virtual instruments available, to be used by keyboards and hardware samplers, or in software samplers. You'll find free samples available on Internet or in digital music magazines, but the best samples are usually commercial products, so if you don't have a keyboard you'll need to spend some money.
I can't say 'The winner in the 'Samples contest is...', because It depends of the sound you're looking for. Sometimes you'll find the sound you want on the CD of a digital music magazine or in one of those expensive samples pack used by professionals.
If you have enough money you can check some of the products of Big fish audio, Eastwest or Vienna. Probably Vienna is the most used orchestral sample pack, but it's quite expensive for most of the people, so it's usually used only by professionals. A cheaper option could be the Big fish audio products.
Ok, here ends the first chapter. I hope you liked it.
See you in the second chapter: 'Inspiration exists, but it has to find you working'