As you are probably aware there are a variety of UNIX platforms e.g. SCO,AIX,HP,linux, etc..
what you probably want to do is learn the fuindamentals of UNIX , the structure of it , what the layers are etc..
from this you'll get an understanding of how UNIX works , becuase most UNIX OS are similar just different commands .
You can start with Linux / SCO which can be installed on an intel based server.
You must have a task in order to learn UNIX i.e. what will be your end goal, i.e. networking, setup printing, Disaster recover , user setups, scripting etc..
Also have look at this site and search on net if learning UNIX
HTH