I don't know many people who are hiring event planners for baby showers... it's something most people can do on their own.
However, why do you want to do something that you know nothing about?
would I need to:
book a venue for them - no. You need to make sure the venue meets your clients needs and the contract terms are acceptable.
buy decorations = ????? they are hiring an event planner, they expect to have CUSTOM decorations... you will buy the components and put them together. you would be responsible for setting up the event. you can hire someone to do this, but oyu are responsible for it.
book cakes, catering = you work with the person giving the event to determine the menu and who will be handling the delivery.
Also, do you recommend to buy stuff (decorations, plates, silverware, etc..) before starting a business?
No. Every event is going to be custom. You are going to make decorations and rent the other items.
How do I find out what are the typical rates in my location? If you don't know anything about being an event planner, why do you want to be one.