When in first position, you should be seated with your hands at the widest part of the handlebars. Your shoulders should be relaxed, your chest should be open, and there should be a slight bend in your elbows and your knees. Be sure not to hunch forward or to slide to the front of the seat. This position should feel comfortable and can serve as your home base during a ride.
Feel free to adjust the resistance so you can pedal at a comfortable cadence. Clockwise increases the resistance and counter-clockwise decreases the resistance. Always remember that if you need to stop the flywheel quickly, you can press the resistance knob straight down, and that will brake the flywheel.
Start by increasing the resistance by turning the resistance knob clockwise until you feel some effort when you pedal. Once you feel your resistance is high enough to require some effort when pedaling, raise yourself out of the seat. When in second position, you should rise out of the seat with your head up, chest open, shoulders back and down, and your hands gently holding the bend of the handlebars.
As you stand, your hips should be just over the pedals. You should not be standing straight up or leaning far forward. Your core muscles should naturally activate to support your body without your hands bearing any significant weight. Before an instructor cues to second position, you will likely have a fair amount of resistance on your wheel to ensure stability. Typically, this position is used for lower speed jogs, not high-speed sprints.
Start by increasing your resistance once again until you feel an effort when you pedal. In third position, you are also out of the seat, but, unlike in second position, your hips should be over the pedals and your hands should be in between the bend and the end of your handlebars. Keep your chest open and your gaze forward. Maintain a slight bend in your knee and make sure that your shoulders don't creep up towards your ears. A strong core will offer support. If this position is not comfortable, slow your pace or return to your seat.
Standing takes some time, but you'll get used to it and your stamina will increase. Third position is mostly used for steep climbs, to accelerate and increase power as you pass someone in front of you, or as a break from seated position.