I want to start by saying that you don't need to do a full-face of makeup if it's not your style. Several reasons I say this is because you'll have to invest in all that makeup and it's probably not going to be used after the big event. Buying at the drugstore is much less expensive than shopping at the department store, but even then drugstore prices for one of everything can cost a lot too. At department store prices, the price of all that makeup could easily be the same as the cost of an evening dress and at the drugstore, a pair of shoes.
Okay, back to lessons.
If you want to take lessons at a counter, figure out what
you want to perfect before you go, such as how to perfect your skin, or do a basic, beautiful eye. There is a lot to learn if you try to do everything at once at the counters, and this can create a type of confusion and resignation that leads you to buy your whole look there. If you keep it simple and focused, you'll be less overwhelmed and a bit more confident to shop elsewhere for what you need. Also, go during the week when it's not too busy. Weekend artists tend to be hit with more customers and you don't want a rush-job. It's good to bring a picture to help articulate what type of look you're interested in too. You could also try having a counter do your makeup the evening of the big event. I don't like to recommend this because I had a lot of prom girls come to the YSL counter I worked at only for the purpose of getting pretty for that night. No big sales there *sigh*, but it is a dirty trick if you want to pretty up for cheap.
For the record, I own a lot of high-end makeup as well as drugstore products. High-end is fabulous stuff, but much of what's in the drugstore is also excellent too. A $27 mascara is gorgeous but I'm rocking a $6.99 one from Maybelline and it gives me the most beautiful lashes! But I'll never part with my Laura Mercier lipsticks and glosses - I really love the feel and look. Bottom line, buy what you are really impressed with and if the purchase makes sense in your budget and expected uses in the future.
Another suggestion - check out the bookstores for beauty books that offer a wide range of looks. Bobbi Brown, Trish McEvoy come to mind immediately. Their books give advice for looks that are clean and simple all the way up to full glam. Bring a tablet and take notes for what you will need. Also books aren't there to pressure you into buying unneeded items. You might be able to find books like these at your local library, as well.
And last - you could always book a salon to do your makeup. It can get pricey and you won't walk away with products, but the option is out there.