Honestly, I think you need to talk to your parents/guardian about this. I'm not sure how old you are, but you seem rather young to be exchanging money via mail with someone you are unfamiliar with. Your folks should be able to help you out.
You do not send cash in the mail because, like others have said, some people will steal it. Bar the serial number, cash is untraceable. Therefore, people who find cash will often steal it, as they can readily take it to the store and spend it. The same is not true for say, a check, where they must risk themselves to go attempt to cash it. In addition, a check indicates who it is made out to, and if their identification does not match that name, then they will be unable to cash the check, and they may get arrested by the police for attempting to do so.
Another example of cash being stolen is when people drop their wallets or purses. Oftentimes, the cash will be the only thing missing if the person is lucky enough to get their wallet returned to them.
Please talk to your folks about this. You seem to be engaged in some material that is a little bit beyond your understanding.
Make no mistake about it though, we all used to be like that