Your title means a little different than the post, so ill address both:

How do I buy credits?  Go to the Manage menu -> then Purchase.  Here you can put in a dollar amount that you would like to purchase via most major credit cards/paypal.  You get the credits a few seconds later, the transaction is now done.

Investing is a touch different.  The above is buying credits and getting them instantly and its done.  Investing is something we do to mutually help each other in the long term.  Say you invest 120 dollars, every month for 12 months, you get refunded $10 back via paypal, as well as you get 10 credits given to you as interest.  So at the end of the 12 months, you have your 120$ back as well as were given 120 credits as a thank you via 12 monthly 10 credit chunks.