What is golf etiquette?

The unwritten courtesies that keep the game safe, quick and fair: do not distract others, keep up with the group ahead, and look after the course.

What should I do if I hit a ball towards someone?

Shout 'Fore!' loudly and immediately.

Push the edges of the mark in towards the centre with a pitch-mark tool or tee, then tap it flat. Do not lever the middle up.

If your group is slower than the one behind and there is space ahead, wave them through.

Golf · Guide

Golf runs on courtesy. Most of it comes down to three things: be safe, keep up, and leave the course as you found it.

Be ready when it is your turn: pick your club while others play.
Keep up with the group ahead, not just ahead of the group behind.
Play a provisional if your ball might be lost, to avoid walking back.
Let faster groups through if you are holding them up.
Leave the green as soon as you have holed out; mark your card at the next tee.

Repair your pitch mark, and one other if you can.
Do not walk on anyone's putting line, the path between their ball and the hole.
Stand still and quiet while others putt, out of their line of sight.

Replace divots on the fairway, or fill them with the sand provided.
Rake bunkers after you play out of them.
Keep buggies off greens and tees, and follow the club's rules on where they can go.

The rules themselves are in golf rules explained.

What is golf etiquette?: The unwritten courtesies that keep the game safe, quick and fair: do not distract others, keep up with the group ahead, and look after the course.
What should I do if I hit a ball towards someone?: Shout 'Fore!' loudly and immediately.
How do I repair a pitch mark?: Push the edges of the mark in towards the centre with a pitch-mark tool or tee, then tap it flat. Do not lever the middle up.
When should you let a group play through?: If your group is slower than the one behind and there is space ahead, wave them through.