usually easier to stick to longer term.
you don't need to join a gym or buy expensive equipment to have fun and make exercise a part of your day!
start by identifying a time that fits into your day to day routine ie before / after school or after dinner. by finding a convenient time it is harder to find excuses.
slowly build up more vigorous exercises ie start with 10 minutes and gradually build up to 60 minutes most days.
be realistic with your physical activity goals! 