Let's continue! 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 Digital Audio Workstation (DAW). There're a lot of DAWs available. This is a software application that includes all you need to produce music: a sequencer with the ability to record/play audio/MIDI, a mixing table, plugins and effects and other music production tools. Some of them are Cubase, Ableton, Reaper, ProTools, Logic, Cakewalk or Sonar. Some are quite expensive, but you can find some free (Cakewalk) or almost free (Reaper). There is a free version of ProTools also (which is the DAW used on most hi end recording studios), but there're other options like Emagic's Logic fun, a free version of Logic, its famous DAW. Anyway, most of the them works in the same way and have similar functions. So surf the web to learn more about these products, check your pocket to see if you have enough money to buy your favourite or download some of the free ones.
2nd. Libraries/Samples. If you don't own a keyboard with builtin sounds you'll need a sound library (usually in the form of a plugin containing one of more virtual instruments) or a sampler with sounds to produce the sound of your music. Unless the virtual instrument uses an algorithm to generate its sounds it will also use a sample library to generate sounds, acting like a sampler with some options to modulate or make the sample sound realistic (using transitions between samples, etc).
In case you don't know there is basically 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 you can use them as base 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, as each sample usually contains just one note of the instrument. Put all the samples of all the notes of that instrument together and you can create a virtual instrument, which you can use in your songs to produce the sounds of that instrument. For example, if you got a virtual instrument of a violin, a sitar or an orchestra's string section you can use it and play that instrument virtually with your keyboard, for example. In the past the most credible instruments samples were drums/percussions, as these instruments usually don't have a lot of space for expression. But nowadays virtual instruments are so good they can mimic string, brass, or other kind of instruments on a pretty realistic way.
There're A LOT of libraries of virtual instruments/samples available. The best libraries are usually commercial products which are not cheap, so if you don't have a synth/keyboard with onboard sounds you'll need to spend some money. I can't say which library is the best, because that depends of the sound you're looking for. If you have enough money you can check some of the products of Big fish audio, Eastwest, Spitfire audio or Vienna.
Ok, here ends the second chapter. I hope you liked it!