Saying yes is much easier than saying no. Most of the time though we say yes because we don't want to hurt feelings or we want to be nice. What is the point saying yes if you don't plan to do what you are asked? Saying no is mostly seen as rude, even though it makes perfect sense some situations. This may not be a big deal when its friends and family asking you for a favor, but sometimes answering a yes or no question can determine our future.
At the work place
When I was still going to school, I had a job in a warehouse as an "associate" as they like to call it. My job was to get sold items from the store, bring them to the warehouse, wrap it or box it, load it in the customers car when they came to pick it up, and check off the order from the list. It was a very physically challenging job. I was part of a 10 men crew who was always running around and one guy was sitting on the computer doing inventory. When the "computer guy", as we called him, takes a day off, someone has to replace him. One day my supervisor asked me if i knew how to enter the order in the computer. My answer was, Yes of course. He gave me a ram of orders. There was probably 200 orders that were not processed. I sat on the computer and had no idea which program does what. But at the end of the day i had manage to enter a couple orders. The next day i showed up early and finished them all.
I could have been honest and tell him I have never used it, and we could wait for the computer guy and he would finish it the next day because it wasn't that urgent anyway. But because I say Yes I learned something new. Any request that came along whether it was delivery, inventory, building a bed, or modifying the website I was the man to go to; not that I knew how to do any of that but I would take the risk to learn and maybe screw up sometimes. You would be surprised how simply apologizing sincerely can get you out of trouble.
Saying yes, helped me learned much more in a year than those who always said no because they didn't want more responsibility. Who would have thought I would learn about programing excel macros in a warehouse.
What do you say when you don't want to do it.
When you are asked for a favor you don't want to do, say No. Simple as that. Saying no is considered rude, especially when you don't give an explanation. But just think about it, when you say yes, you are not expected to give an explanation, the same should go for no. Here is my suggestion, if the reason you don't want to do something is laziness or being scared of the unknown, you should probably say yes.
Today I still say yes most of the time, with exceptions to when there is absolutely no gain for me in it. I feel like I would be missing on too many opportunities otherwise. Things get easier as you say yes, and it will help you gain confidence when tackling something new.
When you have said yes enough then you can have the luxury of saying no. But until then, it's a journey of great discoveries.