Your baby is growing up and no longer can you call your child's room a nursery, but now it's time to make this space into a real kids room. You may need to purchase a few pieces of furniture such as a bed to make this transition but the real overhaul will be in the theme of the room.
You may not have to get rid of everything from your nursery but instead work to blend the pieces. For example, if your baby had an antique white armoire in the nursery, there's no reason why you can't incorporate this piece of furniture into your "big kids room." Don't try to match the color if you are using a different brand, it will never happen. Instead choose something that compliments this like an iron bed. Even if you own a simple white dresser, don't think the new white bed you buy will ever match. Instead pick a colored frame like pink or navy.
Besides the bed, if you have the space, you may want to add a table and chair set for your new little explorer. This table will become a main attraction in which your child will explore all different activities from make believe, to puzzles, to writing.
At this time in your child's life you can decide on the longevity of the room. If you choose furniture that is theme based like a race car bed, you'll need to make a another change in a few years. If you pick furniture that is simple then you will be able to keep the same furniture for a long time.
Create a theme with everything other than the furniture to make your baby's room become a big kids room. For a boy you may think about sports, space, a safari, trucks and cars, pirates etc. For girls you could do flowers, ballet, princess, fairy tales,etc... The types of theme are endless. What is most important from a decorating perspective, is that you use this theme and stick with it through and through. Incorporate your idea in your bedding, area rugs, decals, accessories etc... Suddenly you will see your baby's room become a kids room filled with the things your child loves.