Credits are used to bet in the vBookie sections of the forums. The vBookie is a betting system used on the forum to bet on fights. You use your credits to bet with, not actual money.
You can use the credits to do things such as donate them to others that need them, you can use them to buy sigs in graphics section(you need an upgraded membership to the forum to do this), as well as bragging rights for having a high amount of credits.
The green do is the reputation system. The more green you have, the higher your rep on the forum. The more red you have(instead of green), the lower your reputation is.
You can see how much rep you have in the control panel. To give rep, use the little scale icon under your green dot.
Also, welcome to the forum.
“Anna, my strong, terrifying Anna.” -Cas
"You lack the requisite spine and testicular fortitude to study under me" -Elodin