Every store should have a sizing chart that will help you out. For starters, use a measuring tape and figure out your waist, hips, and bust. Each brand is going to be different in sizes and they also don't account for other parts transwomen have to deal with (feet and shoulders), so trying stuff on always helps. For example, I have to wear a size 3 skinny jean simply because my feet don't fit through smaller leg sizes, and I usually have to wear medium or large shirts because I'm a bit tall (5'9") for some brands and my shoulders are slightly broad.
So basically, there's no reliable conversion chart. You have to measure and go by brand sizing charts.
Shoes are easy though. Womens shoes generally run a size and a half smaller.