The entire point of Monopoly, how you win the game, is to corner all the cash and make your opponents run out, or go bankrupt. So the process is to buy property you land on, get a monopoly (yes) of that color group and then improve them with houses and hotels.
Acquiring a Monopoly of All Properties in a Color Group
Before you buy any houses or hotels, you must own all the properties in that color group.
For instance, you can only buy houses on the yellow color group—Atlantic Avenue, Ventnor Avenue, and Marvin Gardens—when you own all three properties and none is mortgaged. You can buy a property when you land on it.
Once you own a monopoly of a color group, you can make improvements on your properties with houses or hotels anytime during your turn or even between turns of your opponents.
After you own all of one color group you may buy houses from the bank for that property. The price for each house is on each property card.
A key rule is that you must place houses evenly on your property. If you buy one house and put it on one property, the next house you buy for that group must go on another property, and so on. If you buy three houses at once for a color group with three properties, you must put one house on each of the three properties rather than, say, three houses on one property.
The more houses you have on a property, the higher the rent you charge when an opponent lands on it. But the rule about placing them evenly keeps you from loading one property with houses while the others lag.
When you have four houses on each property in a color group, you can buy a hotel.
You pay the bank the price listed for hotels on that property card and give the bank the four houses that are on that property.
You can buy hotels one at a time and leave houses on the other properties in the color group. Only one hotel can be bought for each property.
You get the highest rent when you have a hotel on a property, so this is the ultimate strategy for the winning the game. If you can accomplish this on several properties or groups, you will be a regular J.W. Marriott, and you will have a great shot at winning the game.
Running Out of Houses and Hotels
The game has only 32 houses and 12 hotels. When the buildings have been purchased and are in use in the game, the rules say that no more houses and hotels can be bought. This building moratorium can end if a player goes bankrupt and return houses and hotels to the bank, chooses to sell them back to the bank, or buys a hotel an returns the houses on the property to the bank.
Once the bank has these returned houses and hotels, they are available to be bought by any player for their original price. A bidding war can ensue if more than one player wants them, with the buildings going to the highest bidder.