Hi!
There are two ways of doing what you want:
1/ using a high level (and maybe graphical tool)... I'm sure there are several such softwares, but i must say i don't know any of them.
2/ using tc (for traffic control i believe) which is the low level shell tool for configuring quality of service under Linux.
You'll find an introduction to tc at
http://lartc.org/howto/lartc.qdisc.html
I would recommand as well having a look at the other chapters of LARTC, that's definitly good reading.
By googling, you'll find several other doc on tc...
