An introduction to computer audio
You major choice is the operating system you are going to use for your audio PC.
If you are not very computer savvy and totally blank to computer based audio I do think mastering a new operating system and mastering computer based audio is for masochist only.
Stick to the operating system you are familiar with.
The differences between operating systems sound quality wise is explained here.
Your second choice is your media player.
The best one is the one who fulfill your needs.
But how to specify your needs if you are new?
A good starter is WMP on Win or iTunes on Mac.
Once you have built up some experience you have a better insight in what you are missing.
Sound quality wise I do think drivers are more an issue than media players.
This is explained in more detail here.