I don't know almost anything about networking, i have searched but have not found anything useful, if anybody could give me some ideas, or point me to a how-to and mention the specific steps needed in my case I would be forever(or a month or so) grateful. Here's the deal:
I have one Sagem F@st 908 E2T ADSL modem (cant post URL, Sagem page has limited info) that connects to my Trendnet TE100-S5P+ switch (trendnet page has pretty pictures of it) which should have 2 Debian computers behind it. As far as i have read a router would connect to the Internet permanently and do some dhcp magic so i wouldn't need one computer playing server, but routers cost too much i think, so do i need to set up one computer as a gateway or something? I bought the switch because a friend of mine had basically the same setup, but he could configure his switch with a browser, so that makes it a router/switch? Please feel free to correct me if I am mistaken.

So that's just a standard ADSL modem, it will provide you with a single ethernet or usb port. As it itself won't have an IP address in itself, you won't be able to use that as the gateway. so as you thought, one machine would need to be a gateway. You can't run all three devices through a single switch though, well at least i've been told "weird things" happen if you do. so the gateway machine will need two physically seperate NIC's (or one NIC one usb connection if you think you can make USB work. to make one machine be a gateway, you just neede to set upi pforwarding on it via iptables. someone has kindly written [wiki]A basic firewall configuration suitable for a gateway/nat[/wiki] using [wiki]NAT[/wiki]in our wiki, with debian speciifc details, so that should do you fine.

and you come out of it with a spare switch to flog on ebay.