Full rate plans, individual, family, and just data for existing plans
are here:

http://www.apple.com/iphone/easysetup/rateplans.html

$20 to add iPhone Data (unlimited), visual voice mail (unlimited),
and 200 SMS messages per month onto any existing rate plan.

See how to activate an iPhone here:

http://www.apple.com/iphone/usingiphone/activation.html