How to approach getting a raise?

TES

I'm a certified dental assistant, been part of the practice over 2 years. We had a situation with our front desk , one girl was writing fake reviews... I told the boss what was going on. girl got let go , Then office manager asked if I wanted to take on front desk and go full time since I'm only part time assistant. I agreed , it's been 3 weeks of training. haven't asked for a raise because I wanted to see how I liked the position and if I wanted to continue. well 2 days ago the office manager/treatment planner was let go. now the big gm is taking over , and now in responsible for front desk , assisting and treatment planning and am now full time. were having a meeting tomorrow and want to ask , he has shot me down last time I asked saying there isn't enough funds. I make $13.50 an hour and want to make at least $16. How do I make sure I get the raise I feel I deserve , they are now let go of 2 paychecks so I know money cant be an issue. they are just greedy. help ?!