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.
Before you buy any houses or hotels, you must own all the properties in that color group. So 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.
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. You can buy houses one at a time, but you must place them on properties evenly. 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. Or 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.
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.
When You Can Buy
You can make improvements on your properties with houses or hotels anytime during your turn or even between turns of your opponents.