How to fix sign in error

At first the issue was "Unable to Purchase" for any new apps. So I thought "Oh let me just sign out and then sign back in, tht might fix it."

However, that plan did not work because now I cannot sign back in. I've searched for a couple days and some place said it was because my account is empty (money wise)

whixh is is another issue and if you could help with that I'd appreciate it: I purchased a $0.99 pack for Spider-Man Unlimited, did not receive my IAP product and now my account has no money in it, which shouldn't be the case either.

So I guess my overall question is "How do I put some money into my account so that I can log back in to the App Store and iTunes Store?"