Let’s start with a few basic “ettiquette” rules.
Most importantly, keep pace. That means, know when it is your turn to hit, be ready when it is your turn, and hit. Generally, whoever is furthest from the hole is up. And whoever has the lowest scor on 1 hole has honors - hits first - on the next. So that is the basic ettiquette to avoid hitting out of turn. But if mediocre golfers (most of us) strictly adhere to this, it can cause delays. You should at least know how to play “ready golf” - essentially, whoever is ready, hits.
When you hit your ball, watch it all the way until it stops. Pick out a landmark - a tree or something - and walk right to it. If you hit your ball to the right and your partner his hers to the left, don’t stroll over to her ball chatting with her, and just assume you will be able to find your ball when you cut back across the fairway. There is little more frustrating than a not-very-good golfer repeatedly hitting lousy shots, and then saying “Where did it go?”
Most golfers have a pre-shot routine, if only a practice swing or two. But if you have an involved routine, you don’t need to wait until the guy before you has hit before starting it. Walking up to your ball, you should be thinking about what club you might hit. And you should have it in your hand before the last guy is finished hitting. It really slows things down if you do not even start thinking about club selection until it is your turn to hit.
Be aware of where the next hole is, and leave your bag where it will not delay things. For example, if the next tee is behind the green and to the right, don’t leave your bag at the left front of the green. Little things like this can really slow things down and annoy your partners and the folks behind you.
What is best is if you can find a group of golfers who know the game, but also know how to enjoy themselves on the course. Playing regularly with a group like that is the easiest way to learn the ettiquette an how to enjoy the game while improving. Most of all - have fun. Remember, it is just a nice walk in the park.
Re: clothes, personally, I always sorta shy away from dressing too nicely. IMO, if I see someone wearing the latest most expensive golf fashions, they’d better be a damned good golfer. If they aren’t, they look kinda silly, as tho they are dressing up and playing a part. Believe me, a knit polo shirt and shorts/slacks/skort are just fine, and for the first year or 2, you’d be better off spending your $ on lessons. Just my opinion.