Salary Negotiation Advice

Hello! So I need some advice as to how to ask for a raise. A little background:

I was hired by an EI company last summer. For about two months, I was training (going on visits with an OT who had worked there for 5+ years). When the training period ended, the three other OTs quit unexpectedly leaving me with a HUGE caseload. I sucked it up, put my head down and got to work. I was the only OT for about three months. During this time, they gave me a stipend of 1500. I recently found out that the OT who was hired after me makes about 3,000 more per year than me, despite having less experience. I've met or gone over my required productivity every month. Any advice as to to how to go about asking for a raise would be GREATLY appreciated. Thanks in advance!