Adding custom fields to a WP database

Admittedly I’m new to WordPress, but I found a theme I like and am ready to get rolling. Now, as with any theme, not all the options are there for me as I’ve got an existing movie-review web site. So my question is this: Let’s say I want to add something “new” (as in not included with this theme) to my admin. Let’s say it’s the running time of a movie (it’s not, but just to illustrate my point). Is there an option in the WP Admin that will allow me to do that or will I need to go into the code and allow for a new field as well as add a new field to the database too?

Sorry if that’s confusing, let me know if I need to further clarify.

Thanks for any and all help you can offer!