Reality is no area is 100% safe over time, some may be 98-99% safe, but for starters, check crime reports for an area including all of Fairfax County and the DVM area at
https://www.crimereports.com/
Look at areas of the county/Northern Virginia with no bus service or only commuter rush hour bus or limited bus service. Obviously avoid areas around Metrorail train stations. Some criminals from the inner city come out here, and if frequent transit service, especially at night, they use that as their 'getaway vehicle' and now with GPS tracking on smartphone on most transit around here including Metrobuses and Fairfax, Connector, they know when to show up at the bus stop minutes before the bus arrives. See links below.
https://www.fairfaxcounty.gov/connector/sites/connector/files/Assets/Documents/PDF/FFX_SystemMap_2019_web.pdf
https://www.fairfaxcounty.gov/connector/schedules
https://www.wmata.com/schedules/maps/upload/VA-System-Map-2018.pdf
https://www.wmata.com/schedules/timetables/all-routes.cfm?State=VA
If an area of the county where frequent, all day, late night service, and service seven days a week, with any good amount of moderate and low income housing, those areas will typically have more crime and where can get robbed. Examples of such areas include Hybla Valley, Bailey's Crossroads/Culmore, Lincolnia, and central Annandale. I am not making this up, as have a close friend who does crime raids for a federal government agency in another part of the USA, confirming my thoughts on this.
Areas with no bus service or very limited bus service, rush hour only and poor infrequent bus service outside of commuting hours, and/or higher income housing, lesser crime and less likely to get robbed, but not 100% guaranteed. Such areas include Clifton, Great Falls, Franklin Farms, Oakton, etc.
Personally, I fought the county to limit weekend transit out in western Fairfax County including Chantilly, Centreville, some west Fairfax areas, when it started in mid 2016, for the fear of the crime it can bring, so we have less than what they originally proposed. My neighborhood has zero weekend transit, closest buses close to a mile away. Buses only run to around 8pm on weekend nights with bus frequencies at only 45-60 minutes. Off peak ridership including midday/late eve weekdays and weekends, the majority of the Fairfax Connector buses out here have fewer than 10 riders and many less than five (personally seen from late eve commuting from Vienna Metro worked late at office here and there, plus car breakdown over several weekends), in one of the highest automobile ownership areas of the county. Lighter used buses, waste of taxpayers money, not to mention the pollution when no or 1 or 2 persons on the bus, where now relatively cheaper alternatives such as Lyft or Uber ride share, but that's a separate topic altogether.
My best wishes for the OP's safety. I do not know what delivery services hire those under 18 or 21, but any delivering more expensive food such as Panera vs. cheap pizza, may or could some reduce the likelihood encountering a customer who could rob you, plus better tips, or perhaps a small local family owned restaurant who does deliveries. Also Lasership in Tysons and Chantilly looking for drivers to deliver packages, no contact with customers hardly, but their pay practices are questionable nor know what they age requirements are.