Pretty basic really for this one, just get hotels on your properties for this one.
You must have both (brown/dark blue) or all three (3) (every other colour on board) for a monopoly before being able to get houses.
Price for houses is such:
Brown/Light Blue: $50 each
Pink/Orange: $100 each
Red/Yellow: $150
Green/Dark Blue: $200
The cheapest hotel will cost $450 on the brown properties (under Official Monopoly Rules of equal building), you don't need to build each property of colour into a hotel, just one.