There is not one specific spot you should stand in to return serve.
Each player has a certain area that they feel most comfortable returning serve from. Some players stand right on or inside the baseline. Many stand just behind the baseline and some even stand far behind the baseline. Players also adjust to stand more towards the doubles alley or towards the middle of the court.
It is very important that the returner adjust up, back, side to side, etc., according to their preferences as well as the tendencies of their serving opponent. If the player you are facing has a great out wide serve on the ad side, stand closer in and towards the doubles alley. If their strength is serving more down the middle, stand more in that direction.
It is also a good idea to start in a certain area, and as the server tosses the ball, move slightly towards the area you would like to cover. Make sure you are set (split step) as the server is about to make contact with the ball.
The position that you return serve from can give your opponent second thoughts about hitting a certain serve and give you the advantage when starting the point.