How do I signal I'm not ready, or stop play for a hinder?

Not ready: before the score call starts, signal by raising your paddle or your non-paddle hand above your head, or by turning your back fully to the net (Rule 6.A.1). Once the score call has started, those signals no longer count, but verbal calls such as "stop" or "wait" are still recognized (Rules 6.A.2, 6.E).

Hinder: any player may call a hinder when something not caused by a player interferes with play, such as a stray ball, a flying insect, or a player from another court. The rally is replayed (Rule 9.C). Permanent objects such as fences, posts, lights and ceilings are not hinders.

If you stop a rally for any other reason, it's a fault against you. The only other exceptions are correctly pointing out a server or position error, or asking for an incorrect score to be corrected before the return of serve (Rule 9.A.1).