Tri-suit or 2 pieces. Don't try to put a T-shirt with your back wet because you will spend several minutes.
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If you can afford it, get tri shorts and a tri top or a tri suit. If you can't, I would wear speedos (I prefer the long shorts) underneath and have a LOOSE shirt waiting a T1. Just pull the wetsuit off and go. Just remember you'll be wet, so if you plan on putting anything on other than shoes, practice, and practice wet. I got my wet arm caught in my sleeve and had to have a friend pull my shirt down. It was a mess!