To sell hamburgers in Monsterville, you have first to earn the respect of the locals. You should first select a shop and start there (there's various, because it's a city with many buildings and business). In the first level you'll have 3 customers that are very patient, because they know you're learning. In the following levels, no one will be so understanding.
Click and drag and drop to play. A very detailed tutorial will explain you how to proceed: click on a client to know his order, then move the mouse over it to know exactly what ingredients are required to prepare it. Start by burning the burger: you have to turn it when one side is ready, and use a spatula to take it from the fire; then combine the bread, cheese, etc. You only need to use the mouse to play. It may be a bit difficult to learn when you start, but once you get it it's very easy.