1 - Why are you throwing an event in the first place?
Is it your birthday? Is it a cat's birthday? Is it a work dinner party or a charity event. Nailing down why you're hosting an event will not only help you focus where to look for a venue and what to do but it will also let you know who your guests will be and knowing that is key to any event.
2 - What is the goal of your event?
What do you want to achieve with this shin-dig? Maybe it's everlasting popularity or maybe it's to get in your boss' good books. Whatever your reasons are, always write them down before planning an event and at every big step or decision check that the direction you are going is inline with those goals.
3 - What is your budget for the event?
Nobody likes having one when it comes to events but unless you're Puff Daddy's daughter planning a 'sweet sixteen' we all have one. Start off by planning your event to land comfortably in budget as practically all events have unseen coast, "wait the DJ needs equipment?". and if you don't have an unforeseen expense you can throw your remaining budget behind the bar. :)
4 - What's nifty ideas do you want to 'borrow' from other events?
Have you attended any events recently that blow your mind? What made them so special? The best events are always completely unique from anything the guests have been to before, but is there anything you've seen at other events that you can develop on and make your own? Mixing, matching and merging ideas will always bring you to a fresh new place.
5 - What will the guests need?
Is your event in a Motte and Bailey Castle? If so you're probably going to need a drawbridge for your guests. When planning an event never stop thinking about your guests. The goal will always be for them to have a great effortless time, so whatever you think they may need, do your best to supply it.
Now that you have all that sorted, all you have to do is decide what to wear.