Both are simple to acquire~
Poke: Just rp normally, you'll get it normally, either by battling, rewards, contests etc~ you'll get it all one way or another.
Credit: You'll get it normally just by posting~ but you can do some other stuff to get credits, like selling items or converting.
Credits can be converted to Poke, and vice versa - but the credit shop is generally more expensive and it is harder to come by than Poke, other than means of conversion and/or increased activity.
The exchange rate is 10:1 meaning for every 10 poke, it's 1 point and for every 1 point you convert to poke, it's 10 poke. Get it? Good.