how to make the Ai consider obstacles that are created in game (like walls) when making their paths,this can be done with navegation mesh and steering actuator?
Walls are no obstacles. They should be not part of the walkable area (navigation mesh).
When using the build-in navigation generator walls and holes are excluded from the navigation mesh automatically. The navigation mesh will leave gaps that ensure the character will not touch the walls.
The path finding will take place on the navigation mesh only. Any other area will not be considered. Therefore any target to move to has to be placed above the navigation mesh. Otherwise it is not reachable.