You will only study at your best if you are physically fit and mentally prepared. Some simple suggestions are below:
Keeping physically active, also benefits your brain. Find a form of exercise that you are happy to do and include it in your daily routine.
Organise your day and make sure you allocate time for activities other than study. You will need to be flexible and adapt the plan to fit circumstances each day,, but most of the time you should be able to follow your plan.
If you have been sitting for a period make sure you move around during the break
avoid using screens - a break should not be spent on the phone!
make sure you get a drink
Sunlight and fresh air are good for your body, make sure you include time outside the house in your daily routines.
Make time to talk to friends and family.
Do NOT overeat or consume lots of junk food, but make sure you eat plenty of nutrious food and drink plenty of water
Your body needs sleep and it likes that sleep to be predictable - keep to a consistent routine.
There are lots of people you can ask for help: parents, friends, teachers, school counsellor,... do not be afraid to discuss your problems, they all want you to do well.