First off, as simple as it sounds, wear antiperspirant. Not deodorant, you want something that's labeled as a clinical strength antiperspirant. Apply it liberally but be careful if you have sensitive skin. Baby powder can also help if you're extra sweaty but it's kind of a mess.
It's also important to wear a sweat resistant undershirt which I know sounds odd but if you want to wear a super-thin white dress shirt that will keep you cool you're going to need an undershirt or else you're basically going to be a one-person white T-shirt competition. A good undershirt can help wick moisture and at least keep you comfortable if not cool.
You will eventually stop sweating once you get acclimated to wearing a suit as long as you're not standing in the sun. It'll be really uncomfortable at first but once you wear it all day I promise your body will eventually adjust.
