To start using rune pouches you must first do a small miniquest, started by talking to the Zamorak Mage in about level 7 wilderness, just north of the Grand Exchange.
After you do this miniquest you will be given a small pouch for free, and can use higher level ones. To get the higher level pouches you can kill monsters in the Abyss, which you will now have access to, or you can wait until 50 runecrafting and then buy them at the Runecrafting Guild.
Hope this helps. :)