What should I say and do for my first time at a MAC Counter?

They can be rude but you never really know until you go up to them. Also what you want to say is fine, but i wouldn't say you are on a budget so are only going for one, if you say this they may only show you one lipstick, without saying this they may show you a few and you can see which one you like better and just get the one :) If they don't help you and are rude, they will probably just let you try them on yourself, i know in mac they keep an alcohol based sanitizer what you can use on lipsticks before trying them on

They're generally not stuck up at all. I've been in a few times to the one in Leeds. I have even sent my boyfriend in to pick me something up....they laughed at the fact that he went in in shorts and steel toe cap boots with phone in hand to show the exact picture of what I wanted. They're normal people.

Your approach sounds fine. Just be confident, they are human just like you. You are a customer, regardless of your age - they shouldn't be rude to you. They should ask you if you need help and you can just reply "Yes, I'm looking for _______"

I am not ashamed, I go right up there and tell he lady, "I've never been here before, nor do I know anything about maykup, can you help?" I do that in drug stores too, and it really makes my shopping experience easier.