It helps to have your bike in front of you as you read through these steps the first time. On cheaper bikes, buy a whole new wheel.
Bike tires wear out just like automobile tires, only faster.
They are also susceptible to the insidious decay that sets in over winter when active gear molds in the garage. Replace the tire if the treads are badly worn, the underlying cord is visible or the sidewalls are cracked. Inflate tires to the maximum pressure listed on the sidewall. Let them sit an hour and check the pressure. If it drops more than a couple of pounds, replace the tube.