Change air filter:
Get a new air filter for your mower of the exact same style as the old one. It is preferable that you change your filter every year. Open up the cover that holds the air filter with screws, install the new filter and screw back.
Replace the blade:
Use a screw driver to open up the blade of your mower. Take it to a home store and get the exact replacement. Bring it back to replace the old one. A new blade will get you visibly better results at mowing.
Remove the old spark plug. If it is rusted, use a penetrating lubricant try again after some time. Pull it off with a spark plug wrench or deep socket. Install the new plug by turning it with your hand. Remember the new spark plug should be of the same model as the old one.