Blank canvasBefore you get started it's a good idea to clear out the room. This will give you a better idea of what you are working with. You will need to consider just how spacious the room is, for smaller rooms go for lighter colours. Light colours reflect light and light makes spaces appear larger. If you decide to use a darker tone you'll need good lighting. Don't be afraid to try something different, decorating your child's room is a great opportunity to get adventurous. Try a feature wall with wallpaper or a block colour.
Final touchesThis is where you really should get your child involved. What are their favourite characters, colours, quotes, movies, celebrities and what inspires them? This is meant to be their space after all. You'll be able to insure that they don't go overboard. You want their space to be inspirational but to still be easy to organise. You want them to be able to keep their own room tidy.
In the end all you need to do sit down with your child and discuss together what should be included. It's their sanctuary so they should have a voice about the issue. The room should include somewhere for them to sleep, relax and study. Finally, set a budget and stick to it. You can a lot of great things with a small budget, work with what you have. Not everything has to be brand new.