I've made a deal with a person over email. He needs to send the payment online (around $3000), what is the quickest way I can make this happen? I need to be able to accept this transfer by tomorrow.
PayPal, if you both have accounts. You can transfer money instantly from one Paypal account to another. All that is needed is the other person's email address. But your accounts must already be created and verified if you need it done by tomorrow. So if either of you don't have one it won't be done tomorrow via PayPal.
I don't know the details of your deal but do be cautious of scams and don't give out your personal banking information to a stranger. That is one of the advantages of PayPal, it is safer because your banking information is not given.
There are other companies with services comparable to PayPal but PayPal is the biggest and most widely used.