If you are only just starting a channel, you cannot monetize at all.
As of April 6th of this year, a new rule went into effect. Channels must now surpass 10,000 lifetime views before they are eligible for monetization. They then must apply for approval to the YouTube Partner Programme.
The candidate channel will then be reviewed for the following:
1. Copyright compliance
2. Community Guidelines compliance.
3. General YouTube Terms of Service compliance, including publicity and privacy rights.
Channels hoping to make money off unlicensed copyright content, certain types of prank videos, compilations, religious or political commentary, dangerous stunts, e.t.c., will be automatically denied monetization under these new rules. A denied channel will be given the general guidelines which might have drawn the denial, but no specific information.
A denied channel owner will be given a 30 day cooloff period to find and remove the media which led to the denial; after which period, he or she may reapply for monetization approval.
Once approved, advertisers bid to place ads against your videos. You do not choose what ads will appear; you can only choose what will not appear by blocking certain ad types in your Adsense Dashboard.