Years ago I faced the same issue. I decided to tell my boss and he actually supported me with advice and mentorship (he ended up convincing me not to quit, but that's another story...)
The first question that comes to my mind is: Do you really need to quit? Having an income while you moonlight is a huge advantage, as you can survive until your start-up is sustainable (and then quit your day job)
Second: Do you have enough savings to survive a year on your own? If not, keep your current job, build your savings and then quit.
Third: Are you in good terms with your immediate manager? If yes, then do tell him/her -- You might be able to even negotiate an alternative deal, like part-time, contracting or something else. If not, then tell him/her only if asked. In any case, it's certainly better to learn the news through you than through someone else.
Obviously, what I wrote in this post does not apply if you and your employer have a conflict of interests. If that's the case, I'd talk to a lawyer.
The first question that comes to my mind is: Do you really need to quit? Having an income while you moonlight is a huge advantage, as you can survive until your start-up is sustainable (and then quit your day job)
Second: Do you have enough savings to survive a year on your own? If not, keep your current job, build your savings and then quit.
Third: Are you in good terms with your immediate manager? If yes, then do tell him/her -- You might be able to even negotiate an alternative deal, like part-time, contracting or something else. If not, then tell him/her only if asked. In any case, it's certainly better to learn the news through you than through someone else.
Obviously, what I wrote in this post does not apply if you and your employer have a conflict of interests. If that's the case, I'd talk to a lawyer.