Make sure instruments are kept in cases and in a safe place when you're not playing them.
Do not leave stringed instruments near a radiator or other sources of heat.
If your instrument goes out of tune, don't try to tune it yourself (this invariably results in broken strings!) Your instrumental teacher will happily tune the instrument for you at your next lesson in school, or alternatively have someone else help you with tuning – someone with a digital tuner or pitch pipes may help here.
Most importantly, try not to let others play your instrument, especially younger brothers and sisters. It's your instrument to look after at the end of the day.