Don't tell people you are writing a book. Or that you have wrote a book.
If you tell people these things you won't be greeted with eagerness or excitment, you will be greeted with defensiveness, the "What do you have to write a book about?" attitude.
No really. You will.
Even people who are suppose to support you, like your family, won't do it.
I can tell you right now what they will say....
"What are you going to do with it?"
"Do you know how hard it is to get published?"
"You don't know how hard it's going to be to do anything with it."
Here are a few answers you shouldn't say back because people don't like to hear them:
1. I was thinking about using it as a means to prop up my coffee table.
2. Oh, so have you ever written a book?
3. Now that I am finished I was thinking about burning it
4. Well, I will be sure not to mention you in the dedication.
5. Shut up.
If on the off chance you find someone interested in what you wrote you will soon discover that they are just itching to compare it to something already written.
"Oh, wasn't that already done? Didn't Stephen King write a short story similar to that in 1981? Really, your idea just sounds so familer."
Here is my advice to you.
Just don't mention it. And when you do get it published. Gloat gloat gloat gloat gloat.