How do you make your grandpa let you play computer everyday

Perhaps make a deal that you do your homework before you play.

It depends on why he isn't letting you do it now. First be sure you know why he isn't wanting you to do that. Then come up with things that will fix the problem he sees with your using it everyday in some other way.

For example, if he doesn't want you using it every day because he wants you to spend time doing other things instead, like homework, ask if he will let you use the computer for a half hour for every half hour you spend doing homework. Or if he wants you to spend more time outside and being physically active for your health, then negotiate how many hours of that you need to do for an hour of daily computer use. If it is because he can't afford it, then ask if you can do some work for him to help pay for it or get an after school job to help with the costs. You MUST keep up your end of the deal if you use this approach.

Perhaps he is concerned about the type of things you are using the computer to do. Does he think the games you play are too violent or too mature for you?
Ask him to watch or help you play so he can be reassured that it is not something inappropriate. Change to something else if it is inappropriate. Or add on some more educational or learning uses.

If he wants to use it when you want to use it...then try to find something you'll both enjoy doing on the computer together or create a schedule that you both can stick to so you both get your time.

Some older people don't realize the educational benefits that using a computer can provide. Ask him to help you ask or answer some WikiAnswers questions and things like that so he is aware that you are not just wasting time, but are actually improving your knowledge and helping others by the use.

Don't whine and complain, ask to talk to him about it calmly and find out what he sees as the problem with your using it everyday and then ask him to help you solve that problem so it is a win-win situation for both of you.