Accounts and select your account.
You may have to authenticate to perform the next step.
Click on the Login Options
Next to Automatic login, which is probably off, click the drop down arrows and choose your name.
Enter your password and apply the changes.
That should do you.
I've set up a new iMac and it asks me for my password every time I turn it on. I've looked in all the places I can think of to bypass it and all I've succeeded in doing is that it now asks for both my name and my password. How to fix it?