If your lawn mower is been in winter storage for a long time and you have not applied regular maintenance to the engine, it will be a struggle to start up the lawn mower engine. I understand that it is very difficult to start a lawn mower after winter storage because it happens with my own petrol lawn mower. So in order to get it going again you will need to buy a carburetor cleaner, that is supplied with a little tube that acts as a spraying nozzle.
Once you have the carburetor cleaner, you will need to then locate the air filter that's inside the lawn mower engine and remove it. Once it is removed the carburetor will be exposed, then using the carburetor cleaner, spray it directly onto the carburetor. Once you have done that, then replace the air filter and prime the lawn mower 6 times and attempt to start it up. This method always works to start any petrol lawn mower the first time, for a lawn mower that has had a hard time during the winter weather.