Contact an Air Gun repair shop near you. Look on the Web under "Airgun repair or Air Gun Repair" and send it to a location close to you.
It very hard to help you if you don't state the name, model...
The reverse of how you took it apart. I would look on the web to try to find drawings of how to assemble the BB gun. Try looking at the company that made the BB gun first, some offer drawings of the...