Trail to Premium

How do you change your accout from trail to premium. I have 60 credits.

The first 60 credits are for free trial (15 days). To become a premium user you need to buy more credits. To buy credits just click on the credit information at the top of the page. There you have several possibilities to get more credits and become automatically premium

what are those possiblities please. Is the premium credits a one off purchase or its continues in order to play the game?

Please check out the wiki page on the payment system to answer these questions. :)