What do I need to know before watching 3D for the first time

Are 3D glasses needed?

Yes. While not as bad as the early days of 3D where viewing without glasses meant red and blue headaches for days, the video will look slightly off. Note that many films have scenes, not the whole film in 3D, so you may still enjoy the experience overall.

If so are they provided?

Yes. I am not aware of any theater that doesn't provide glasses. The glasses will typically fit over prescription eyeglasses without modification.

If they are provided would a better experience be had, by purchasing 3D glasses?

You won't know until you try them--if you intend to make a habit of it it might be worthwhile, but you run the risk that the technology used for the effect will change and you will have to repurchase your glasses.

So I am I clueless, what do I need to know before walking up to the counter and purchasing a ticket to a 3D movie?

You may experience eye strain and motion sickness, and possibly discomfort due to poorly fitting glasses. However it will be a novel experience, and hopefully as enjoyable as seeing a movie in 2D.

If they are provided would a better experience be had, by purchasing 3D glasses?

I wouldn't bother. There is no one pair of glasses you can buy that just "does 3D".

There are many different ways to display 3D, all of which would use slightly different glasses. So basically, if I was hardcore enough to want to do this, I'd have to check every movie beforehand to make sure my glasses were compatible with how that movie's production company decided to encode 3D on the print of the movie sent to my theater (or to pick the right 3D glasses from my stockpile I had to buy to support every 3D tech in use in the movie industry).

Glasses are required, but are provided. I don't think buying your own will be worth it—the technology could change over time and may be different between theaters.

Many movies are provided in 3D, but not all scenes from a movie are shot in 3D. Some CGI effects are slightly different between versions.

This usually does not make a difference and the movie looks great. However, I prefer Rogue One in 2D because the CGI effects to make actors look young again didn't translate gracefully to 3D (maybe the details were just overlooked by the editors, I don't know).

Some people report motion sickness-type feelings. It isn't common.

Personal preference: I bet you'll have fun and love it, but nowadays, I always prefer 2D movies: I don't hassle with holding and wearing glasses (especially the weird feeling of having glasses on over my normal glasses); I feel I can "take in" the movie better (see and understand the whole picture at once); and I don't worry about messed up CGI details.

You should be aware that while most people enjoy 3d movies, a significant minority either can't see the 3d effect, or have an adverse reaction to it. Statistics vary, but about 10% of the population is stereo blind and as many as 1 in 4 report eyestrain, headaches, or motion sickness. A recent study suggests that only a third of people have no problems at all viewing 3d movies, and that about 20% of 3d movies have technical errors that are likely to cause problems (e.g. scenes with the left and right images swapped).

Don't let that put you off, the chances are you'll be fine, but if you do feel unwell don't feel you have to stay in the cinema and suffer. The worst experience I ever had in a cinema was trying to watch Avatar in 3d; I wish I'd left after five minutes.

If you have a vision impairment, you're gonna have a bad time.

The magic in 3D cinema technology relies on relatively good vision in both eyes. If you have poor vision in even just one eye, you will barely, if at all, see any 3D effects and walk out of the film with a headache.

For people who don't know if their vision impairment is going to get in the way of a 3D film, a good test is to ask yourself if you could/can see the 3D effects on a Nintendo 3DS.

One other thing to keep in mind with current 3D technology is that it depends on stereoscopic vision—each eye sees a slightly different (flat) image, which your brain then integrates into a single, 3 dimensional image. That might sound obvious, but it means that some folks with particular kinds of vision issues won't be able to see the 3D effect the way it is intended (one of my children has strabismus, and doesn't enjoy 3D movies as a result). Some individuals may not even be aware of having a minor problem until they try to watch a 3D movie and discover that it doesn't work for them.

So I am I clueless, what do I need to know before walking up to the counter and purchasing a ticket to a 3D movie?

Other answers already mentioned most of what you need to know, but there is something else you might want to consider. If you're not comfortable wearing glasses (even if you're just slightly uncomfortable) or simply not used to wearing them, the 3D glasses will make you pretty uncomfortable throughout the entire movie. I saw the Last Jedi in 3D last week, but I left with a pretty big headache due to this.

It doesn't take away much from the enjoyment from the movie, but it is something to keep in mind. I personally tend to avoid 3D movies because of this.