The equipment question is the one that stops most people, and it is the smallest of them. Flat-soled shoes that grip, clothes you can move in, water, and a racquet, which many clubs will lend you for a first session. Buying before you know whether you like it is the wrong order.
The first hour is not a lesson in strokes. It is rallying: the ball goes over the net, comes back, and goes over again. That is the whole skill at the start, and it is more satisfying than it sounds. A coach will fix a grip in ten seconds that would take a month to find alone, but you do not need one to begin.
Expect to be tired in an unfamiliar way. Tennis is short bursts and hard stops rather than steady effort, so an hour on court asks something different from an hour of running, and the day after tends to find muscles you had not been introduced to.
The etiquette matters more than the technique and takes five minutes to learn. Wait for a point to finish before walking behind a court. Call the lines on your own side, and give the benefit of the doubt away from yourself. Send stray balls back when the point is over, not during it. None of that is written down at the club; all of it is noticed.
General guidance, not a research finding.