You get credits by posting and creating topics.
5 pts per each new Poll, Topic, or Reply.
There is a bonus if you get a certain number of people to reply to your topic (but I won't say how many replies or the amount of the bonus. That is for you to find out ...
Once you get enough credits you can go to the IBStore (on the toolbar at top under the logo) and buy stuff with them like ability to pin topics, etc.
You can store credits you arent using in the bank, and they will accrue interest (but you have to go to the Bank screen and click Collect Interest every couple of days unless you buy Auto Collect)