This is mostly written about group classes or led sessions, but it may apply to other situations as well.
What to wear
Any athletic clothing is fine. Wear trainers just in case (or if you prefer), but it can often also be done barefoot.
What to bring
Nothing is necessary; all props should be provided, unless otherwise specified. A water bottle is a good idea.
If you have issues with grip, you can try using chalk. Some people wear lifting gloves for wrist support and to reduce calluses. Personally I don’t usually do either, but I’ve seen plenty of people make use of it.
How to prepare
Arrive early enough, ideally rested and hydrated. Maybe have a snack a bit beforehand. The session should involve sufficient warm up, but I like to do additional wrist and shoulder stretches and exercises to get fully ready.
What to expect
Usually the session starts with a short ~5 minute warm up, which involves wrist mobilization, full body movements (such as walk outs, push up or lunge variations) and core exercises (plank, dish/hollow hold, etc.)
Main part: 3-5 sets of 5-12 repetitions of some number of the following: push ups, pull ups, chin ups, dips, knee/leg raises, squats and lunges. It can also involve handstands or other hand balances. In a more advanced setting people might also work on skills such as L-sit, lever, muscle ups, working with rings etc. The exact exercises partially depend on the equipment available.
Calisthenics is more about control and precision than speed. I’ve seen it run similarly to a HIIT-style class before, but it’s relatively rare.
While exercises are usually performed individually, it’s common for students to spot and help each other.
Typically the session ends with a very short cooldown, so I like to do additional stretching and rolling out at home.
Grip – if you’re not used to hanging from a bar, your hands and forearms might be the first part to give. This will get better with time. You could also use chalk during the session, or practice grip strengthening exercises at home. The skin will probably also be affected and you will also quickly develop calluses.
Feeling weak – if you’re not very fit, it can feel like everyone is ahead of you when you first start. Majority of women and many guys can’t do a pull up in the beginning, and that is perfectly normal. Keep in mind a lot of people might just have more experience and have already done months if not years of training. If you keep doing it consistently, you will progress too.
After the class
It’s nice to help put away any equipment you might have used, such as bands or boxes to stand on.
As mentioned before, I like to stretch out more after the class. Foam rolling or other kinds of massage can help with soreness and tightness as well. If you haven’t already, eat some food involving protein to help with muscle growth.