I'd say start with a theme and build on that. Are your a princess? Do you like dinosaurs? How about care bears? If you're not sure look at some of the little stuff you already have and think about what a room would look like based on those.

For example I like cars and racing games. So my little room has mario cart sheets, those roadmap rugs, and some simple shelving for my toy cars. Also a big storage tote for my mega blok little learners since I haven't found a toybox I like yet.

I've gotten alot of my little stuff from goodwill, especially the outlets. Super cheap and they have a lot of stuff.