If you used the "Sign Up" link in the upper right hand corner of the screen, you would have chosen a password during the sign up process.
If you started creating a family tree online by entering your first and last name and the first and last names of your parents from the home page at MyHeritage.com, then you should have received an email with your password in it.
If you didn't receive that email, first check in your junk mail or spam mail folders in your email inbox. Still didn't find it? Go to MyHeritage.com and click on the "Log In" link and then "Forgot Your Password". A new password will be sent to you.
In the case that you still haven't received any password, please let me know.