How do I calculate a 401k plan?

I just started about three weeks ago at my new job. I earn 30,000 and was told by the controller that a 401k will be offered to me in detail after three months from when I started. So, I'd like to figure out on my own (if possible) how the defferals work. In addition, if I leave after, say, a year or two, what happens to my plan? Will it be frozen until I cash it in and unable to receive further payments or can I continue with it as is?