A wedding planner is hired to make sure that everything leading up to the wedding goes smoothly and that nothing is forgotten. It is therefore important that you share things like what your vision of your wedding is so they know what you are looking for.
Wedding planners are people that help organize and coordinate the activities and tasks needed to execute your wedding. A wedding planner has experience with planning weddings as they have done this multiple times, as oppose to a lot of couples who only do it once in their life, so that is one advantage with hiring a wedding planner to help plan your wedding. Of course another advantage is the time aspect; if you don't have time to plan your wedding; a wedding planner does as that is their full time occupation.
You will likely be asked what your budget for the wedding is. So make sure you have an idea of this so that you can discuss options and work within this framework and avoid any unpleasant and costly surprises along the road when working with a wedding planner.
Even though you have someone else being the main one responsible for tasks getting done leading up to your wedding you will still need to be an integral part of the planning and decision making, so the more information you can give your wedding planner the better. You are going to need to have meetings and check-points where you go over what has been done, what is coming up and make any decisions as needed. It is up to you, in discussion with your wedding planner, where you want the line to be drawn; do you want your wedding planner to pick out your wedding flowers or do you just want them to make sure the flowers gets delivered or anything in between. Be clear up front with how you would like the relationship to work. Different wedding planners may work in different ways so if you are not sure how you would like this relationship to look ask them to describe how they usually work. Now as with any closer relationship, choose your partner wisely. Your wedding planner will be someone you talk a lot to and will need to rely on during the whole time leading up to your wedding so make sure that the wedding planner you pick is someone you get along with and someone who's skills and expertise you trust. If you have a recommendation, see if you can meet with the wedding planner or just talk to them over the phone to see if the chemistry is right.
Sometimes places like hotels and other types of venues may have a wedding coordinator on their side, someone that will coordinate the dinner (food, beverages etc) if that is being provided by them, the timing of dessert, the cake etc. This is something to ask about when you research wedding venues just so you know what they will handle and what you will need to take care of. Although, this is not really the same as having your own wedding planner that looks after all aspects of the wedding, it is a big helping hand to help you deal with a lot of the details coming together smoothly on the day of your wedding.