Just be friendly, approachable and open to learning. I live with the theory that if I listen to what people say, even though I might already know it, the worst that will happen is it refreshes my memory and I might pick up something new.
I'm on a first name basis with the people in my local tack shop, and that's what keeps me going back. A friendly person who you can chat to is what brings you back.
I have never worked at a tack store.. So all I can say is that as long as you're fun to be around, have good people skills, and sorta know what to do when it comes to sizing tack and riding gear for humans, it seems like a really fun job to me.
I guess just expect really weird people to come in sometimes, and know that you'll have to just deal with it the best that you can. Goodluck!