You might begin by looking at your closets and figuring what you need. Do you have just one that needs improving or do you have a whole houseful? Are your closets old and smelly? Do you think you need a whole new closet design or do you think all you need is a better way to organize things?
You then need to decide whether you can do a closet makeover yourself or whether you want to have a professional come in. Most professionals (the most famous is California Closets) do a great job and they have lots of closet design ideas but they are quite expensive. If you have an older home, it might look silly to have expensive modern closets in a home that looks totally different. But if you have a newer home and the budget to go along with it, you might want to investigate what a professional company can do for you.
If you decide to try the makeover yourself, you first need to empty your closet and do a thorough cleaning. You will want to rip the wallpaper off the closet shelves if it has wallpaper and pick a nice fun color that goes well with the rest of the room. Paint the closet walls or replace the wallpaper with a better one.
To upgrade a bigger closet, you might look into some lighting options. You can make some cool closets with the creative use of lighting such as track lighting. You can also add furniture to your walk in master bedroom closet if it is really big.
If you are doing things yourself, you should check out all the home improvement stores in your neighborhood or ask around to find out who sells closet organization systems. Pick the one you like and make sure it is the right size. You will then have a totally new closet that you will be able to keep clean and neat. It will make you much happier every time you open your closet door to see something that is not embarrassing!