We're not in NoVa, but in general in Va, you have to send a letter to your local school board stating that you will be homeschooling. It's commonly called an NOI, Notice of Intent. It's not mandatory to do this until a child is 6, BUT, you must notify them if you have 5 yr old that will not be attending K. For a 5 yr old, you can send in a NOI to homeschool K or a simple letter stating your child is not ready for Kindy so they will not be going. When they are 6, you decide if you NOI will be for K or 1st.
My boy is only 5 this yr, but I'm told starting at age 6 a yearly evaluation of work or testing must be done, you choose which you'd rather do.
There is a lot of info on homeschooling in Va on the VaHomeschoolers Organization website.
http://www.vahomeschoolers.org/law/
HTH and welcome to VA!