You can hold on or not hold on. Most treadmills have programs, so after you are used to the unit, it's good to switch off and do different things so your body is always guessing. Make sure the bike you use has pedal straps, so you can use the muscles on both sides of your legs Make sure your position allows for almost complete extension of the leg. If the seat is too high, it places stress on the lower back, and can result in crotch pain. If the saddle height is too low, you won't get good muscle balance and might injure your knees.
Most people pedal too slowly with too much resistance.
Exercise bikes allow for much more flexibility than any other piece of equipment. While walking and reading, it's easy to get tripped up. Reading, headset, television, visualization. I try not to have people get too complicated doing too many things at the same time, because I don't want anyone to get hurt. Just do it slow, and get some reps, get the burn.