First, decide your approximate guest list. Actually make the list, so you get an idea of how many people. Then figure out your budget.
Then you can choose the date. But you cant choose a date until you choose a place. So first start looking at a place to have your ceremony and reception. This will give you an idea of when to have it, and how much planning you need to do. Some places will book out a year in advance. While other places will be free a few months from now. If you are having a large wedding and need catering, DJ, etc. it will require more planning than a simple cake & punch reception.
So start with the basics, and it will take shape from there.