Shogi is played with a few small rituals. Nobody will insist on them online, but they're part of the game, and this site has a way to express each one.
At the start: お願いします
Before the first move both players bow and say onegaishimasu お願いします — roughly, "please, let's play". In a game here, the reaction bar offers it for the first two moves.
During the game
Play at a steady pace, and don't leave a lost position sitting on the clock. The reaction bar has a few friendly sayings — いい手 (nice move), おお! (wow), しまった (oops) — and nothing that could needle your opponent. Reactions are a fixed list: no typing, so there's nothing to moderate, and each player sees them in their own language.
You can mute your opponent's reactions for one game, or turn them off everywhere in your social settings.
At the end: 負けました and ありがとうございました
The losing player normally resigns before the mate with makemashita 負けました, "I have lost" — here it is sent for you when you press Resign. Then both players say ありがとうございました, "thank you for the game" — or お疲れ様でした. And if it was a good game, offer a rematch.
Draws and impasse
Offer a draw only when the position really is level, and at most once in a while. An impasse can only be declared when both kings are in their promotion zones (see the impasse lesson).