Add variables to track the scores:

Name these player_points and enemy_points

We need references to:

the paddles 

the ball 

the interface.

Select the LeftGoal in the Scene Tree

Double-Click the point_scored() signal

Rename the function to _on_enemy_scored

Repeat for the Right Goal...

Select the RightGoal in the Scene Tree

Double-Click the point_scored() signal

Rename the function to _on_player_scored

Create a function to reset the state of the game

After a goal is scored:

Paddles return to start locations

Ball returns to the centre

Ball speed set to 0

Paddle speed set to 0

Re-launch ball

Code the _on_enemy_scored signal

Update the points for the enemy

Update the User Interface

Reset the game state

Repeat this when the player scores.