Hy. Sorry for my english. I had installed debian testing on my pc. I made a netinstall, first de core sistem, then a basic kde4, and video driver. I don't know how to set my sound card, i have a onboard card, my motherboard is gigabyte ga-965p-ds3. Also i don't know how to set my network to share files, my internet provider provides dhcp for one connection but at home i have 3 computers, one with debian, one with windows xp and one with windows seven.I'm using a switch with dhcp to connect the computers. Also i don't know what codecs to use for movies and mp3.

I can't find details on which audio chipset is on the board, but to find out, in a terminal (look for something called 'terminal' or 'konsole' in the menus) enter:
and hit enter, and you'll get a list of all the devices on the pci bus, including audio chipsets. Your next port of call is then to open up Synaptic and search for packages using the model number string. For example, my lspci includes the line
so I'd search for packages including the text '82801h' or 'ich8'.

There might not be any, in which case you want to see if the manufacturer offers them on their website, or check the alsa website.

How to do this depends which files you want to share, and to whom. To share files with a windows PC, in the same way as Windows PCs do, you want to set up samba, which is non-trivial but perfectly doable. There are howtos for this all over the internet. To share files with the internet over ftp or http, you need to install an ftp or http server, and configure the firewall on your router appropriately.

This depends on which movies and audio you want to listen to. Mplayer certainly used to maintain a codec pack of sorts that covered just about everything.