if you are getting error msgs you should copy them down and simply punch them into steampowered.com
I have done this for you:
Here Yah GoIf that ISN'T the right error msg, make sure you copy down the correct one and punch it in there.
If you are still having problems then come back here (or go to the Steam forums) and give us(or them) system specs (focusing on NIC/Router/Connection/firewall) and exactly what steps you have taken to try fixing it.