You need a credit or debit card or one of those prepaid Visa, MC, American Express gift cards.
Then just go into your account (top left, next to the Log Out button) and buy credits. Use the credits in the Pro Shop to obtain new clubs. You'll see where everything is when you click ACCOUNT.
Also, be aware that if you use an American Express credit card (or prepaid gift card) then you'll get double credits until June 21st.
I recommend the R9 and don't forget to equip your clubs after you've bought them.
Good luck!