It's always a bit nervous your first time out and I can understand how it would be even more so for you.
Don't stress too much about it, go have fun, find a role you can play even if it's "hold this trail" or "defend the base". At least you're out slinging plastic. As you get more used to it you can try more to the limit of your ability.
Last big game I played an older, heavier gentlemen carried a camp chair around with him. no one made fun of him for it