In the option window of the tool you used to create a surface, use the pop-up menus next to the names of the generation and rail curves to choose what level of continuity the surface should maintain with its neighbor along that edge.
To open the option window for an existing surface, choose Object Edit > Query Edit and click the surface.
See View and edit the construction history of an object.
Set a manual tangent angle along an edge
How do I use the tangent angle manipulator?
I can’t achieve the continuity I want?
Certain types of continuity are not available in the pop-up menu for an edge?
Some continuity types (Tangent, Tangent Angle, Curvature) are only available on edges that are connected to surfaces.
If it looks like your new surface shares an edge with an existing surface, but the continuity choices are not available, it probably means you accidentally clicked a construction curve, not the edge of the existing surface.
To fix this, delete your new surface, then hide any construction curves that are “in the way” and re-create the surface.