I'm thinking of quitting my job
Why do you want to quit your job?
I don't like to sit for more than 8 hours a day.
It is normal to hate your job, and it is entirely possible to have a career you love. Make sure you have something set up before you quit your current job so that you're not dooming yourself to failure.
I'm going to approach my manager and tell him how much I hate it. I'll get a new job after I quit.
It's alright if you want quit your job but it's rude to tell your manager how much you hate it. You better find some other job first that you think is much more suitable and then quit the current one.