Meeting your girlfriend's parents for the first time is like a job interview, but you shouldn't come dressed in a suit and tie. Instead, you should be wearing things that are nice and simple while ready to discuss yourself and your future ambitions.
The focus is on you, not your clothes in this situation. You want to look friendly and like a normal dude, not like a steezy model who just jumped out of a lookbook. Opt for an oxford shirt, dark, straight-legged denim, and a pair of chukka boots that don't look like you're trying too hard.