When can you say that your site is ready to begin placing advertisments onto?

and when you do begin advertising for others, where do you begin?

I have heard about getting paid for impressions, click thrus, and commissions.

I am more interested in impressions than any other but havent been able to find out much about the affiliate programs that offer CPM.

Can I get some input on this from you guys?
which do you think is better? CPC or CPM?