I would go and personally see the manager face to face and tell him.

I always prefer talking to someone. Face to face in my opinion is the best, I for example have a job where we work nationally so my boss might not be in the same state as me, so a phone call is the next best thing. If that fails (no answer), then I'd go ahead and E-mail.

As far as what the E-mail should say, there's no real right or wrong answer. I just quit my job a month ago for another, had to resort to E-mail because no one picked up their phone and they live in Minnesota and I live in Washington. My E-mail went something like thank you for the opportunity to work at ________, but I got a better opportunity working for a different company. I told them exactly why I quit which isn't exactly professional, but it is helpful to the employer to know why they lost an employee.

Thanks for the advice guys, I think I'll just tell him face to face

Yeah, face to face is not always the easiest but will leave you in good graces. In the past I've done face to face and a written letter.

It's usually best to give a 2 weeks notice.. but if you can't do that, then, like others have said just let him know whats going on.