You play a junction canal card in Phase 1, just like taking a contract from parliament.
Your narrowboats are simply markers to illustrate where you are building canals. Because you cannot have more than two contracts, you should be placing one every time you take a new contract and removing it when each contract is complete.
Therefore, when you choose to play a junction canal card, you place your narrowboat where you are choosing to build. So there would be no reason to place your first tile anywhere else.
Yes, a junction canal is like any other canal except that when you take it you can place the narrow boat in any city/town (rather than having a choice of just two) and that it must terminate as soon as it connects to a second city/town.
The first tile placed must be next to wherever you chose to place the narrow boat, when you took the contract.