Check the fuel level in the gas tank. If the level is too low, the mower will stall from lack of fuel. It's also good practice to top off the tank before you start mowing.
Move the throttle level to the choke position if the engine is cold. This makes the fuel mixture richer -- that is, the proportion of fuel to air is greater.
Move the throttle to the fast setting once the mower is running smoothly. This makes the fuel-air mixture leaner by reducing the proportion of fuel. Use this setting if you're starting an engine that has been running recently.