My parent wants me to do something I don't want to do, how can I tell them no?

You should ask them what you really want to do in your life. Ask them politely not to force you for theirs choice. If they're not going to listen you leave your home and live your life according to you.

First make a list of legitimate reasons why you don't want to do what ever it is. Then sit down with your parent and calmly go over all the reasons you don't want to do it and hopefully they'll accept your reasoning.

Finding a reason it's not a good idea or something else you feel would be a better use of your time & why helps

You're person at all.You have rights to do what you want to do.But our parents care about us.They just want to be the best for us.If they wanted you to do,probably it has a reason.But if you really don't want to do it,say it in a kind way.Don't yell at them,don't fight with them.Try to be sensible.I think it'll help you :)

About a year ago, I'd just moved house, I was having the most ridiculous argument with my dad where I was too stubborn to brush my teeth before breakfast instead of after. My dad was stressed about work and I was angry about moving house, never the less, I decided the best thing to do was just to brush my teeth now, its the best I could do, especially when he works so hard :) Little things like this aren't worth arguing for !

explain to them why i don't want to do it in the nicest way possible without showing any disrespect towards them

Firstly tell them how you feel about the activity. Try and tell them why you don't want to do it. Secondly try to compromise maybe say instead of doing this I'll do this for you instead.

Tell them that you are smart enough to make your own decisions and politely say that you are not willing to do what they expect you to do

Well, if you don't want to do something then just flat out tell them in the politest way possible in which they won't think is an act of defiance. Stick up for yourself and just try to compromise with them.

Just like that, you kindly tell them you don't want to do it, you have free will and cant be forced into doing anything.