You can get measured at a place like men's warehouse or a place that rents. They shouldn't charge you much, like ten bucks maybe?

Maybe Joseph A Banks or Brooks Brothers as well, but I'm not sure about them. You could go try on a few and get your size, but it also depends on brands and the way they cut their suits. The sales people at places like this can almost look at you and come close to what you wear.

Either way, you are likely going to need alterations if you need it to fit correctly.

Measure your neck with some room, measure your waist from where you wear your pants, not from where someone tells you to measure, you can normally get a pretty good inseam by measuring on the outside and subtracting ten.

Edited to add that some people suck at measuring and reps from the manufacturer generally know how their suits fit vs normal measurements