An interior designer will typically work with an architect to remove and or relocate walls or built in features.
If that is not what you are after, you may want an interior decorator, not an interior designer. Sometimes those two professions are interchangeable and cross the same lines, but an interior decorator is typically not qualified to make structural changes.
Like others have mentioned, you will need to set a budget and a wish list. Do you want new flooring, painted or wallpapered walls, new furniture or use some favorite pieces you already own? Do you want a totally different look with new upholstered furniture and window treatments, plus accessories and wall art? These area all questions that will be asked of you, so get your ideas organized. Do you have a piece of art work that you love or another cherished item you want to use in the room? Often an inspiration piece is all that is needed to guide your color choices.
I once decorated a room based on a birthday card the homeowner received because she loved the colors. A good designer will sit down and listen to you. More often than not, people just do not know what they want but they sure do know what they do not want. By listening carefully, the designer can, through the process of elimination of what you don't like, figure out what you do like. A good designer will give you want you want, not what s/he wants you to have. Most homeowners cannot visualize a design plan, so ask your decorator to create an image of their ideas.