Start by finding out from your state's Education Department what their requirements are for homeschooling. Here in WA you need to register with the Ed Dept once your child is of Year 1 age. They will send you a Certificate of Registration and will then arrange a Moderator meeting. The Moderator will come out and meet you to make sure you have some ideas about how you will cover the learning areas. They will come back again in a year to check on your progress and what plans you have for the following year and so the band plays on.
You will also need to think about what kind of homeschooler you will be. Will you 'school at home' and sign up with a distance education provider like SIDE or Southlands? Will you use a boxed curriculum like Sonlight or My Fathers World? Will you piece it together yourself? Will you do any formal schooling at all?
What kind of philosophy will you follow? Charlotte Mason, Classical, Latin Centered, Unschooling, Delight Directed, Unit Studies, Waldorf, Montessori etc. Or will you be eclectic? A google search of any of these terms will produce tons of results to help you get a feel for what they are about.
There are so many different choices to make! It is a fun and rewarding journey :)