The first thing you do is set up an account at Coinbase.
Then using Coinbase you buy your bitcoins which you pay for by credit card. (I had better luck buying
bitcoins with cc than buying
seeds with cc)
Then you complete your order specifying pay by bitcoin.
There's also 25% off first order!
Knowing what I know now (and conveniently being on 420mag) I would probably talk to the
@SeedsMan rep before I even started just to get a convo going for support through that first order.
Here's their info page and if you scroll down there's a thing that takes you to a Coinbase page that has little videos of how to do bitcoin:
Seedsman Bitcoin Info Page