Creating Custom Fields?

I would like to create a custom field called “cust_title” and the value is what I would like the page title (top of the screen) to be.

Something like:

key: cust_title
value: Shop Here, Save More! Everything you need.

Then the <title> in the header.php would be:
<title><?php check for cust_title, display the value?></title>

I’ve read about tags and creating custom fields, but I just can’t seem to wrap my mind around it. Can anybody help, or point me in the right direction?

Thanks!