Depending on how relaxed or formal your workplace is, you could either: 1) Just go in merrily, plonk your bag down and say to the room at large 'Well folks, it's time to go time. I've enjoyed being here but feel that it's not the right fit for me' OR, to your HR dept, boss or senior manager, 2) 'I'd like to tender my resignation as from X (day) as I don't feel that the job is quite the right fit. I need to look at other alternatives where I'm more comfortable. It's not one particular thing, just the overall feeling that it's not making me as happy as I want to be.' Hopefully, that shuts down any persuasive arguments they may have.
I wish you luck...but you won't need it, I'm sure.
Don't expand on reasons, keep it short and sweet. You need to feel right in the job and you don't.
My mum used to hate answering the phone. Things like that and 'confrontational' situations such as handing in your resignation, can be a wall many find hard to climb.
You will do just fine. Deep breath and just say it. 

Afterwards, a reward
or
and 