I'm about 5'8 140 pounds what size is good for me
Small or medium. SB comes long a lot of the time but ghetto fab for sure, but you sound skinny, I'd say small.
Measure your self, then compare to this sizing Small 34"36" (8691 cm) Medium 36"38" (9196cm). I think you are right on the brink of both. If you are going to grow, grab the medium if you are right in the middle of the two sizes.