St. Helena sits on Highway 29 about 30 minutes north of the city of Napa and 90 minutes from San Francisco. There is no commercial air service to the valley — the nearest airports are San Francisco International (SFO), Oakland International (OAK), and the smaller Charles M. Schulz–Sonoma County Airport in Santa Rosa to the north.
By car is how almost everyone arrives. From San Francisco, take US-101 north to the Vallejo area then Highway 37 east and Highway 29 north, or — prettier and slower — cross the Golden Gate and take US-101 north to Novato, then cut east across the Carneros wine district. From Sacramento, Highway 12 west through the Napa Valley or I-80 west to Vallejo and Highway 29 north both work.
By bus, VINE Transit (Napa Valley's regional service) runs route 10 along the Highway 29 corridor from the city of Napa through Yountville and Oakville to St. Helena and Calistoga. Connections from San Francisco require a transfer in Vallejo via the ferry or SolTrans bus. The service is infrequent and impractical for winery-hopping but usable for getting into town. On weekends, a seasonal Napa Wine Bus service runs from San Francisco with stops along the valley.
The Napa Valley Wine Train is not a public transport option but a tourist experience: a restored 1915-era dining car running between Napa and St. Helena with lunch or dinner and occasional winery stops. Worth considering as an alternative to driving if your itinerary concentrates in the valley.