Licences

How exactly do Win2k licences work?
I’m going to Install a new win2k server for the company I work for,finally got the brake I was waiting for to enter the IT field,I’ve been studying for the last 3 years CCNA, novell netware,VB,HTML,A+,etc..
Now the company I work for has a small nt, about 40 users, so I will create an IT Dep.
I will start by upgrading our existing (old box)
win2k server, now, I need some feed back on licencing 30-40 users, How does that work?
Thanks!