I found a wallet and kept the money in it
Why would you keep the money instead of turning it in?
I was really desperate for the money.
If you find money in someone's wallet, you need to return the wallet with the money inside. Regardless of how desperate you are, that money belongs to someone else and if you keep it you'll be stealing, and possibly liable for legal charges.
I'm sorry for stealing
It's good to feel remorseful, but you need to find the owner and give the money back now.