What are some quick ways to raise my credit score?

I'm 19 y/o and I have three major credit cards. I have a credit score of 708 and I'm trying to get it higher. I recently applied for a loan but I need a co-signer. However everyone I kow has bad credit. How can I raise my score within the next couple months so I can get a loan without a co-signer? Or at the very least have pretty high credit. I'm aiming for a 750.