Because there can be such good reasons both for keeping quiet and for speaking up, you have to exercise careful judgment before telling people. One thing to consider is to try "testing the waters" with someone in your family whom you think you can trust. They might be able to help in many ways — for example, preparing you for how others might react, telling you the best ways to broach the subject, providing support for your decisions, etc.
Remember that your atheism doesn't just affect you — by telling others, you are fundamentally altering your relationship with your religious family members. Once, you had that religion in common — it was perceived as a common source of values and tradition. Now, however, you have irrevocably changed that, and many will see it as a change for the worse. If your parents are involved, they may believe that you are also rejecting the morals and values they tried to raise you with — a hard thing for any parent to accept. They may feel they have failed as parents and that it is necessary to try and "win you back" to their faith, putting even more pressure on you.
Perhaps people shouldn't take it personally that you are trying to find your own path, but the fact of the matter is they will, and you should take their feelings into consideration. By that I don't mean that you should stop being an atheist or necessarily continue to pretend being something you are not, but you should take others' feelings into account in the way you phrase things and how much you tell them.
For example, you should avoid attacking your family's religion. Even if you have come to conclude that it is evil, saying so will only serve to exacerbate the conflicts being created and the confusion being experienced by your family. It would also be wise not to take this opportunity to try and deconvert others from your family's religion. They are still dealing with the shock of your atheism — don't make it worse for them just yet. In taking the high road, you will also be showing that being an atheist doesn't make you an immoral and rude person.
Aside from thinking about the feelings of your family before you tell them, there are a number of others things you should consider in advance. For example, you should be prepared for the possibility of some relatives trying to give you arguments for why you should continue believing. You needn't be able to thoroughly refute them all, especially at the time of your announcement, but it would be good if you could demonstrate that these arguments aren't rock-solid and that doubt about them is reasonable. In doing so, you can establish that not believing is itself reasonable and that you aren't being perverse or rebellious for rejecting those arguments.
You'll also need to mentally prepare yourself. You don't want to look nervous and uncertain — that only opens the door for your family not taking your atheism too seriously. You need to be firm and confident about what you are saying. Admit that you don't know all the answers to all the questions, but point out that they don't, either. You have adopted a position that you consider most reasonable — you might change it later on if you find good reason to do so, but in the meantime, this is where you are and they will need to learn to accept it.
Hopefully, you will all be able to use this as an experience for growing and learning. Good luck!
