The first thing you do, before you talk to anyone about quitting, is to make sure you take home any personal belongings you can't live without. Make sure that any coworkers you want to stay in touch with have your personal email address.
You should also bring to work anything the company is going to demand back from you. The worst case scenario is that you are not allowed to return to your desk, have to hand over your blackberry, and are immediately escorted out of the building by a couple of burly guys. (If that happens, don't take it personally).
As for informing the company you're quitting... Personally, I would only do it by talking to my manager face to face. Try to cover the following points
- An exciting new opportunity has come up. Even though I really like working for you, I've decided to leave the company to pursue it
- Therefore, please take this as my 2 week notice
- Here's my list of current projects, their current status and due dates
- I want to make my leaving as easy as possible for you and the department.
- So here's a tentative 2-week plan I've put together to transition my projects.
- Also a tentative list of coworkers who might be good matches to take on these projects
You should then hand over a copy of your resignation letter. Signed and dated, and containing the bare minimum needed to walk away.
Dear [Name of HR rep]
I resign my position as of [date 2 weeks in the future]
Stay focused on helping your boss deal with this unexpected and unwelcome new problem.
Your goal here should be to transition your projects and knowledge as efficiently as possible with the minimum fuss. Leave as amicably as possible (you might want to work with this person again someday)
When your coworkers ask, explain that you have the chance to work at an exciting new startup. Give them the elevator spiel about the project. Talk a little bit about how passionately you feel about it. Try not to go too overboard with it. You don't want your boss thinking you're poaching.
Decline to speak ill of the company you're leaving or any (soon to be ex) coworker/manager/etc.
You might very well face hostility from your soon-to-be excoworkers if they feel you're dumping work on them or if you appear to be belittling them for staying in their jobs. Do your best to remain totally professional and positive at all times. Keep your eye on the goal... quick, efficient, friendly transition of your workload.