Do you have any tips for a first time gym-goer?

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Less than a week ago, I was told that I had a risk of being diabetic, and that I needed to start working out. I've been riding my bike four times a day, for thirty minutes each. That brings my total per day to two (2) hours. On top of that, I've been cutting back on my portions, only eating when I'm truly hungry, and completely cutting out junk food. In less than a week, I've have [surprisingly] lost a significant amount of weight, and you can tell it because my clothes are very, very baggy on me. Anyways, I'm going a gym for the first time ever tomorrow. I have a couple of questions for you guys:

-- What type of clothes am I supposed to wear?
-- What machine do you recommend me to use?
-- What machine do you recommend me not to use?
-- How much time should I spend on one machine?