Just do things like burpees one leg at a time to start with (right leg back, left leg back, right leg in, left leg in) if you're not ready to jump them.
Ditto with star jumps - just do half jacks instead, and keep it low impact when everyone else jumps.
And you probably can do a press up on your knees - if you don't put your knees too far back so they're still under your body, they will support some of the body weight that will eventually be supported by your arms.
Obviously, everyone was a beginner once, and I'm sure they have lots of new people starting all the time, so just keep the weights light, concentrate on your form and learning the moves and lingo, and try to enjoy it!
If you're really concerned about looking out of place or being pushed too hard, you could always speak to the instructor at the start and explain it's your first time and you're not sure what to do.
One of the things I love about trying a new class is being taught my body can do things I didn't even realise it can do - even if you only go once, it's good to try it and see what you think!