I think a lot people have trouble designing a room because they are afraid of making a mistake and they don't know where to start. I know that I still suffer from "The Fear" sometimes even after designing a bunch of rooms that turned out great (well, we like them).
Here's what I do to get started and maybe it will help you to get started on something that you've been unsure of what to do.
1. Pick something!
I read somewhere that if you choose things that you LOVE for your home they will naturally go together and make your house feel homey. This makes sense to me in some ways because your home should be a reflection of you and you should love your home. On the other hand, your tastes can vary between different styles and of course there is the budget to consider... that said, I always start a room with something that I love. The best part is that this should be easy, you know what you love. You don't have to know how it will work right away, just one step at a time.
For the family room, we have that huge sectional that we love. I knew that I needed to work it into the design. For the front hall it was the wainscoting in crisp trim white. For the dining room it will be the curtains that my mom gave me as a housewarming gift for our first house. For the living room it will be a pillow that I fell in love with. Now that you have decided on something you can work from there.
2. Consider existing elements
Now that you have something you love, your room is off to a great start, but its important to take the elements that are not changing (flooring, fireplace, doors, cabinets, etc) into consideration. For our family room we had the fireplace and the bulkheads from the duct work to work around. We took colours from the brick around the fireplace (browns, reds, charcoal grey) and used them in the room. We also managed to build shelving to incorporate the bulkheads and make them part of the room. Working with existing elements is important to making everything work together. If you are in an apartment where you have things that you cannot change, this is especially important.
3. Determine the "mood" of the room
How do you want to feel when you're in that room? For the family room we wanted something fun, bright and sophisticated; we didn't want it to feel like a basement. Since the sofa is dark, we decided that the walls and the flooring should be light (for a brighter feel). Once you choose the paint, flooring and a major item in the room, other things will be easier to choose and will come together more naturally.
Overall in a room you are trying to find balance; you want both light and dark, both solid and pattern, both wood and fabric. Remember that paint is easy to change, pillows are replaceable, art can be changed, reframed, and things can be returned if they don't work. Don't be afraid to make a mistake, you'll never have a room that you LOVE, if you don't try.