Your first step is to find what you love. Your second (and equally important) step is to make sure it will fit in your house! Don't figure it out too late. Remember to measure ALL this stuff before you buy.
You need a frame for reference before you even begin, so draw yourself a little diagram and make sure to note the length of all walls but also measure the usable space. So often there are countertops or radiators or other random architectural features that will prevent you from using all parts of the room. You need to know the measurements of the space that is actually available for furniture.
The length, height AND depth
Everyone knows to measure the length of a potential new piece, but also important (and often overlooked) is its depth (if the back or sides are at an angle or have an overhang, measure from the farthest point). Especially in a tight space, the amount the piece will stick out from the wall is an important factor. You may technically have the space, but if it's blocking a pathway or difficult to maneuver around in a room, it will be a furniture fail.
The staircase and doorway
It's no good buying something that is perfect for your home if you can't get it inside. You need at least five inches on either side of a doorway, stairwell or elevator to even consider a piece and much more if you'll have to turn a corner. That couch cutting episode of Friends was funny, but it won't be funny when it happens to you.
The negative space
That is, the amount of empty space you want around this piece. Remember, just because you can cram something in, doesn't mean you should. You need to walk through your room, place side tables next to your couch, or chairs at your dining table. The empty space is just as important as the furniture itself to make sure your room stays balanced, functional and has great flow.