I have not gotten the gun yet, but i believe that a .45 caliber would be best. I've had a bb gun before so i know the basics i guess..idk
Wait. Are you
buying this gun? For some reason I assumed, perhaps incorrectly, that you might be "borrowing" this gun from someone you know. I think that had something to do with your words of, "I am able to get my hands on...".
A .45 could definitely get the job done. Maybe not as good as a shotgun, but, yeah.
You really should learn all the parts of a gun, how it works, how to shoot it, before trying to use it to shoot yourself. You should be comfortable with it, handling it, loading it, shooting it, etc
first.