Saying no isn't easy, especially when you're a people pleaser. How do you know you're a people pleaser? You put other's needs or wants above your own.
First of all, it's important to stand up for yourself. Especially when it comes to your time. You shouldn't have to give up your valuable time to please others because there is a very good chance they won't do the same for you.
Once you get to the point where you have learned how to say no, embrace it. You can actually be in control of your own life. This is where it can get kind of hard, though. What if you say no to a great opportunity?
The thing is, when you say yes to something you don't want to do, you're saying no to something you love. When you say yes to a job you're not passionate about, you're saying no to your dreams. And when you say yes to someone you don't like, you're saying no to someone you love.
In my experience as a chronic people pleaser, sometimes it's best to say "Can I get back to you on that?" or "Let me check my schedule and I'll let you know!"
When saying no, it's important to be respectful and understanding of the other person's situation. Saying no doesn't always mean shouting out "NO WAY!", most of the time it means saying something like "I'd love to but I can't right now."
Your needs are important. Your time is important. And it's okay to tell the world "No, but thanks."