Lots of people have trouble with maintaining their lawnmowers and often pay for repairs when the problems get too big. This is to help people do it themselves.
Make sure you disconnect the spark plug
This is very important to do first simply just disconnect it.
Drain the oil
Most push mowers will have a drain plug on the bottom just make sure you're ready with a pan to start catching it as it will fall almost instantaneously. Make sure you dont put this oil down the drain as it needs to be correctly disposed of.
Replace the air filter
This can be easily done with a screwdriver. Make sure you see what way the lapels are going on a paper filter so you can do the same.
Remove the blade
This can be done with a bolt wrench. make sure you have the lawn mower tilted for this and wear work gloves to avoid cutting your hands.
Sharpen the blade
Secure the blade with a vice and use a suitable file to run up and down the blade edge. After this reattach the blade.
Reconnect or replace the spark plug
If the spark plug is dirty you can replace it for under five pounds otherwise reconnect it and make sure the wires are not touching as they won't create a spark this way.