Not sure if this was answered already - but if you are back from a break (like me) - at LVL 50 HORDE. You will have to first DO the LEGION open questline (after you get to Dalaran, just take portal back to Origmmar) and then you will see the BFA opening questline next to the mission board. HTH 
what if you are starting fresh, do you get to go straight to BFA? If so, how? Thanks.
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If it’s your first time, you will have to start a new character in the Exiles Reach starting zone. The zones ending storyline sends you to Org/SW and then onto BfA. Any other new character will have the option to either start in Exiles Reach, or their racial starting zone.
this helped a lot! tyvm. Logging in soon.