Tennis has a code that is not posted at the gate. You learn it by imitation, and you notice it mostly when it is missing. None of these rules is difficult, and every one of them makes the game better for the person on the other side of the net.
Arriving
Punctuality is the first courtesy. A court has a start time and a finish time, and whoever is waiting is losing their own playing time. Arrive with room to warm up, and come off on the hour, even mid-game.
The balls
Return balls to the next court when the point has ended, never during it. If a ball comes into your court, stop the point and replay it, without argument. When serving, keep both balls with you: making your opponent wait is a small discourtesy that adds up.
The calls
With no umpire, each player calls the balls on their own side. If in doubt, the ball is good. It is the oldest convention in the game, and the one that says most about the person across the net.
The right noise
You praise a good point from your opponent, you do not celebrate their error. Between points, quiet is the norm. A calm court is the sign of two players concentrating.
Crossing
You do not walk behind a court that is in play. Wait for the end of the point, cross quickly, and thank them with a nod.
The end
You shake hands, thank them for the game, and say something true about it. Then you tidy the court: balls collected, net at height, litter where it belongs.
The rest is presentation. White, clean lines, discretion. See the women's collection and the men's collection.
