If you go in there and they treat you like an idiot then you should pick a new gym. To get ready just make sure and take in lots of water and stretch out before hand. You should probably ask what they want you to bring with you as far as equipment goes. Ive been to a few different gyms and a couple of them made you wait awhile before you started rolling or sparring and the other one just put you in the mix right off the bat. Wearing athletic shorts, a t-shirt your not worried about messing up and a cup is a good place to start with the attire. You shouldnt feel to nervous, a good gym will work with your skill level. Try to remember that everyone in the gym was new to mma at some point. Good luck and have fun.