Moving/Positioning Header in Default theme?

Hi there, I’m new to CSS and I want to start messing around with my header. I want to move my header Title and description, that by default in “Default” theme is centered in the header, to the bottom right or the bottom left of the header. Which CSS code do i modify? I’m not really clear on absolute and relative positions. When I tried to play around with it a bit my whole header/page started messing up. Can anyone help? Thanks.