how to prepare myself to study seriously and fix this problems?

well next year will be my last year in high school, i need high grades to get the collage i wish to join but there's some problems i have, i think it will be better to fix now also to pass this year safely, its all about fixing my concentration, and building a good studying plan.
1-i get lessons and understand them fast but i also lose my concentration fast and get tired.
2-i am addicted to my laptop and my big problem i started book series, so now i day dream more than before that i keep losing my concentration faster.
3-sometimes i feel dizzy because i don't eat much which also effects on my concentration and health.
so how to fix all this,if you can suggest a good studying plan, how long should i study then rest, what can fix my concentration, please share any tips you have.

Make sure you eat breakfast, lunch dinner...and pack snacks for in between. Fruit and veggies preferably. Drink water, lots of it. Low blood sugar and dehydration can effect concentration.

Try to get homwok done when you get home from school. Give yourself 15 minutes to relax but try not to use the computer. Find a spot in the house that is as distraction free as possible and use that as your homework base. Stay off of the computer unless you need it for an assignment.

Work for 30-45 minutes at a time. Take 5 minute breaks. Again, try to avoid the computer or leisure reading until everything is done.

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If I were you I would write a story about you fixing these problems. Learn to day dream about yourself and what you want instead of other people in books.

Find something small to carry around with you to remind you of what you want. A pretty colored rock, a special piece of jewelry, some "talisman"/touchstone.

Remind yourself your body is what carries you around and makes sure you can do the work and not get tired, dizzy, lose concentration. Keep fruit and nuts, easy things around to grab the moment you realize it has been 4-6 hours since you have eaten something. Invent a snack you like with protein, vitamins and minerals (I have a runny, natural peanut butter and use it as dipping sauce for apple slices).

Slowly, deliberately, turn your laptop into something that helps you with your school work, pose questions about your work and do a "scavenger hunt" for answers. Ask questions and write paragraphs in reply; write stories about the questions (or write stories where the questions are asked or the facts are part of the story, etc.; have a main character like yourself who keeps talking about random information (what you are studying) and then the information saves the day, is the answer to the story problem.

There's nothing better for you to do for yourself than to eat healthy meals, snacks, and get 10 hours of sleep a night. Sleep deprivation, as well as failing to fuel your body properly will put you right where you appear to be. mayo clinic

thanks everyone these are really great tips i will follow them and do my best, i think my big problem is also water, because i barely drink one cup per/day that if i drank
thanks again