Theme development: Add functionality for logotype?

I′m developing my very first wp theme and I would like to add functionality for a website logotype. I would like to let the theme-user decide whether to use a text or a logotype file in the header. Can anyone point me in a direction to where I can read about this, or explain for me how to do it? I can′t find any information myself (at least that I can understand – I’m really new to this). Thanks! ??