When the opening ends, it's time for a plan. A plan comes from the features of the position: the pawn structure, open files, weak squares and king safety. Assess the position first, then pick a target.
In this Carlsbad structure, White's classical plan is the minority attack: attacking Black's three queenside pawns with two pawns (a and b) to create a weakness on c6. As preparation, bring the rook to the b-file.