2. People will actually see you as more dependable. When you say yes only when you have the time and true ability to do a great job, then you'll gain a reputation for being dependable.If you say yes to everything, you're bound to do a bad job at everything.
3. When you're selective with your tasks, you'll sharpen your natural strengths. If you concentrate on the things you're good at, you'll be able to improve on your natural talents. For example, if you're a great writer but you're not so great as an artist, you may volunteer to write speeches but you shouldn't sign up to make the posters for your club. Concentrate on your strength and build your skills (and your experience) for college.
4. Your life will be less stressful. You may be tempted to say yes to people in order to please them. In the long run, you're only hurting yourself and others when you do this. You stress yourself out by overloading yourself, and you experience increased stress when you realize you're bound to let them down.
First let's point out the obvious: do your homework.
You should never say no to a teacher, friend, or family member who is merely asking you to live up to your responsibilities. It's not okay to say no to a class assignment, just because you don't feel like doing it for some reason. This is not an exercise in cockiness.
It is OK to say no when somebody is asking you to step outside your true responsibilities and outside your comfort zone to take on a task that is dangerous or one that will overload you and affect your academic work and your reputation.
There will be times when you want to say no but you can’t. If you're working on a group project, you have to take on some of the work, but you don't want to volunteer for everything. When you have to say yes, you can do it with firm conditions.
A conditional "yes" may be necessary if you know you should do something but you also know you don't have all the time or resources. An example of a conditional yes is: "Yes, I'll make the posters for the club, but I won't pay for all the supplies."