Muay Thai huh? I've been doing Muay Thai myself for just over a year now...
I begun my Muay Thai classes with no training at all, and at first I was a bit below average (my kicks were a little awkward, my balance wasn't good). To be honest, you don't really need to take the boxing class before Muay Thai - if anything, you might want to do some fitness training - Muay Thai classes can be exhausting.
Boxing will help with with your punches (and grappling maybe? I'm not sure, I don't know much about boxing), but that's about it. Other than that, you're better off just taking the Muay Thai class straight up. If the gym is full of people that are easy to make conversation with, senseis willing to help and a great Kyoshi, you'll have nothing to worry about - you'll pick it up in no time