It's okay to do weight workouts without weights. Make your hands into fists and go through the motions. 
From there, you can use half-litre water bottles, a can of soup or a book. After that, one- and two-pound dumbbells can come in handy.

If you can't pick up weights

Pushups are easier the higher your hands are and harder the higher your feet are.
For the easiest beginner version, put your hands on a wall about shoulder height. Lean into the wall, and push yourself back to a standing position. When you can do that, find a lower surface, like a table, then a chair, etc.

If you can't do pushups

If you're a total beginner at exercising, you may see other people doing things that may never seem possible for you. But that's not true. Everybody has to start somewhere. Even experienced exercisers feel that they want to be further along than they are now. 
When you start, you only need to do what you can. If you can't run, you can walk and build from there.

Start where you are at

<ul>
<li>Sit in a chair, then stand back up.</li>
<li>Lean you back against a wall and slide down until you're in a sitting position. </li>
<li>Hold onto a countertop or the back of a chair while you do a squat.</li>
</ul>
Don't worry if you can't squat all the way to parallel. You can work on going lower over time.

If you can't squat

Find a video that seems like fun but is just out of your reach. You will find that even if you modify it or rest when needed, you'll come back a little bit better every time.
Remember that your goal should be to do a workout starting at your current level of fitness, not to complete a specific number of reps. Do what you can and rest for the remainder of the time.

If you aren't ready to follow a video

Then walk. Walk across a room, down the block, walk a mile at your speed. Eventually, you'll find yourself walking further and faster. In time you may want to start running. If you hate running, swap it out for something else.

If you aren't ready to run

You've got your bike, you can pedal, but you get out of breath and feel like you can't keep going.
Adjust your workout. Pay attention to the instructor's voice and facial expressions and ignore any specific numbers. If the instructor is talking and seems to be on an easy bike ride, adjust your resistance so you are on an easy bike ride. If she's working hard, change your resistance so you are working hard.