Does your mom or dad like to have a cup or two of coffee in the morning? Does your sister have a cola habit? If so, they can help you with this experiment, aimed at discovering whether caffeine affects the rate at which a heart beats.
The basics of your experiment would be to measure and record the resting heart rate of your subject. You'd then have that person drink a cup of coffee, some soda, or other beverage containing caffeine. You can determine the amount of caffeine they'll be ingesting by reading the label of the soda or coffee can.
Wait for 20 minutes after the beverage has been consumed, and then recheck the resting heart rate. Just be sure that your subject sits quietly during and after the time he or she is consuming the caffeine. Exercising would cause the heart rate to increase, with or without the benefit of caffeine.
Ideally, you could do this with three or four different people, and conduct three trials for each person. Record your results, and see whether the caffeine affected their heart rates.