but I don't know how to start.I mean,which streaming software do I use(is there a good freeware?),which games should I start streaming? do I need a powerfull rig to stream a game?

XSplit is the most popular software, but it is limited to 720p without a license.
Try browsing through here: http://alternativeto.net/software/xsplit-broadcaster/

If you're using Vista or a newer OS, Open Broadcaster Software is a great program and it's free! http://obsproject.com/index

On the site they have a settings estimator for output resolution and bitrate depending on your internet connection speed. You don't need a beefy rig to stream with OBS, it worked just fine with my ancient Q6600 cpu.

If you're on XP, there's a free version of XSplit, although it's rather limited in its settings. Another program is Adobe Flash Media Live Encoder but I haven't used it myself.

You should pick a game that you like and want to share with people. Doesn't matter if it's new or retro. I've seen people stream Might & Magic 1 and SSI Gold Box games and they had a bunch of viewers. Playing a game that you don't like just to get viewers can really put you off from streaming.