Read the instructions that came with the lawnmower.  The advice in this post is important to follow, but there will be guidance specific to your model of lawnmower in the handbook.

Here are a few basic points:

• When you’re using the lawnmower, make sure not to injure your feet by wearing shoes. This seems so obvious, but unbelievably, people do injure their feet by mowing the lawn without wearing shoes.
• This isn’t as important as with some gardening tools (such as strimmers and brushcutters), but wearing thick pants can protect you in case a stone or sharp piece of debris is kicked up by the mower.  It’s not terribly likely, but does happen, and by wearing suitable clothing you can protect your legs from potential cuts or injuries.  In view of this, kick stones and sticks out of the way, and don’t mow over them.  The stones can fly up and hit you, and the sticks can splinter into sharp pieces, fly up and hit you; this can be even worse.
• Ensure you fill the petrol lawnmower with the correct type of fuel and follow the mixing instructions included with the tool.  Check that there are no leaks, which could cause a burn hazard.  Having said this, ensure the lawnmower is properly cared for and maintained.  As well as making sure you’re kept safe when using it, this will help the device last a long time.
• Never turn the lawnmower over when it’s in use and always ensure it’s completely stopped before performing any kind of maintenance.
• If you have pets or know that there are wild animals near by, get them out of the way beforehand.  It’s unlikely they’ll run toward a mower that’s in use, but it can happen.

In general, just use the lawnmower as it was intended to be used – follow the instructions and  make sure you pay attention when using it.