Everyone's life is busy. One thing you can do from the very start is make lists. For example, what would you need for school in the fall (are you in summer school)? Spend some time one weekend and go through your clothes, your shoes, that sort of thing. Is anything damaged? Too small? Then write that down as things to do. Shoes with worn-down heels can be repaired, but shoes that are too small should either be handed down to a younger, smaller sibling (if you have one) or thrown out. If they're in pretty good shape, you might be able to donate them to a local charity.
This shopping list should be shared with your mother or whoever gets you your school clothes, so that you can be prepared and get that done before the fall term begins.
What about classes? Do you have a summer reading list? If you don't, then consider going to the library and asking the librarian what would be some good books for you. Extra reading isn't just fun; it'll also help you to become, overall, a faster reader and you'll usually become better at reading comprehension, both of which are helpful skills in any year in school.
What about school supplies? What do you need? Look back on your older things and also visualize a typical day. If you were always running around looking for a pencil, then put that on your shopping list for school supplies - extra pencils. If you were always losing your homework, consider the security of whatever you're using to carry it. Does your backpack close securely? Are you using a notebook where the pages aren't keeping in the binder? If something isn't working for you, consider other, better solutions. Maybe even head to a drug store or a stationery store and look at what they've got on display. If they're not too busy, maybe even ask the clerk or cashier if they think one brand or organizational system is better than another.
Now consider what a typical school day is like for you, and be honest. Are you up with the alarm? Or do you keep hitting it? If you can't get up with the alarm, maybe you're not getting enough sleep. Maybe try going to bed 15 minutes earlier. If that doesn't make enough of a change in a week or so, try 30 minutes earlier, etc. Teenagers often need a lot of sleep - and they often don't get it. If you can TIVO your shows or watch them on Hulu or Netflix later, then you aren't beholden to a TV schedule.
What about other aspects of your life? If you take a bus to school, do you wait a lot for the bus? Or are you running to catch it? It's generally best to be somewhere in the middle, so analyze why you might be regularly early or late. Are you dawdling over breakfast? If so, then see if there's anything you can lay out the night before in order to speed things up (e. g. take your bowl and spoon out, leave the unopened cereal box on the counter overnight, that sort of thing). Tell the people you're living with that you're doing this so they don't put your stuff away by mistake. Maybe you're slow getting dressed. One thing I found was always really helpful was to lay out my clothes the night before. Not only had I made all of my fashion decisions, I had also done the last once-over to make sure nothing was stained.
What about after classes? Do you do homework? Visit friends? Do chores? Practice a musical instrument? Play sports, either informal or on a team for school? All of these are worthwhile activities but homework has to be a priority (with chores as a secondary priority). You can do homework during a study period in school if you have them, or just after by going to the library or an empty classroom if your school will allow it. Or do it at home. I know things can be distracting, but this needs to be accomplished. For me, seeing my friends was a reward for getting my homework and chores done.
Being unable to find or readily produce your completed assignments can be stressful. Not having clothes that fit can be stressful. Same thing with being a slower reader, or not getting homework done on time, or having issues with your folks because you aren't getting chores done.
But it can all be done! You just need a little forethought. I hope this helps.