If you're a Fairfax voter currently absent from the county due to active-duty military service or living outside the U.S., then your voting rights are covered by the Federal Voting Assistance Program (FVAP).
You should use the Federal Post Card Application (FPCA) to request your absentee ballot from Fairfax County. Download and print the FPCA form (PDF format, ~330 kb). Then complete the form, and mail it to the County Elections Office, Rm. 323, 12000 Govt. Center Pkwy., Fairfax, VA 22035. (If you're not already registered to vote, you may do that using the same form.(
More information for military and overseas voters participating in Virginia elections is available online from Fairfax County and the State Board of Elections.