If you're just wanting a basic server for maybe you and friends to you then a basic PC that fits CS:S recomended spec's, its relatively old so any Core2, 1gb RAM, 8000 series graphics card etc should do.
Software side you'll need a Static IP, you may already have one of these, or you may need to request one off your ISP, Portforward.com also has instructions on setting up a Static IP. Static IP also helps with Team Speak server's.
You'll need a copy of the game and the Source Server ( Steam -> Tools), and there are various config files you can set up to control the game, like setting up VAC etc, so youll probs need to get instructions on this online.
and you'll need a good internet connection. so a 10mb/s connection is ussualy only measuring the down speed, your up speed could be 1/5 of that or less, so you may need to find a good game friendly ISP, or if you're going to host a proffesional server you'll need a proper game hosting ISP.
Don't play a game on the machine that is also hosting the server, its alright if you're playing agianst 5-7 mates, which if that's all you'll be doing I wouldn't bother with most of the above.
Ask this question in the Steam forums for better answers n stuff, but this is my groundwork understanding.
Personally If its a private game with maybe 5 mates I'd run it on my own PC with upto 5 bot's, if we wanted more or were bored of bot's we just used our Uni's game dept's CS:S server. Though we stopped CS:S ages ago in favour of TF2.