Many cyclists make the mistake of not inflating their tires before every ride. Neglecting this quick task can lead to longer periods of pain. An underinflated tire can lead to a "pinch flat," which occurs when your tube bulges out from under your tire and gets slashed by your rim. Over-inflation leads to a loud boom triggered by excessive pressure. You will kick yourself for not airing up if you have to walk home after a flat. Stopping a ride to change a tube is no fun either.
Ascertain the type of valve sticking out of your rim. As the late bicycling expert Sheldon Brown noted, you likely will have one of the two most common types. The Presta valve (called the French or "skinny" valve) is narrow and contains a built-in valve cap at the tip. Schrader valves are just like the valve you find on an automotive tire.
Unscrew the valve cap if you have a Presta valve. This allows air to leave and enter your tube. If you have a Schrader valve, no adjustment is needed. Fit the opening of your bicycle pump's head or nozzle onto your valve stem.
Pull the lever away from you until it locks. The Park Tool website points out it should be 90 degrees away from the pump's hose.
Pump the handle up and down to fill the tube with air. Be sure to consult the sidewall of your tire for the PSI (pounds per square inch) recommended for your tires. On narrow road bike tires, PSIs between 100 and 140 are common. On mountain bikes, PSIs usually range from 40 to 80.
Push the lever down with your thumb at the same time as you pull the pump's nozzle off of your tire. A sudden whoosh or popping sound is common when you complete this quick manoeuvre. As the Park Tool website notes, this is air escaping the pump, not your tire. If you have a Presta valve, be sure to tighten the built-in valve cap so air cannot accidentally escape.
Most modern bicycle pumps work on both Presta and Schrader valves without adjustment. Older pumps and some inexpensive models have a switch that toggles between Presta- and Schrader-ready. Some require the nozzle to be taken apart and a fitting inside the pump head taken out and flipped over to switch between valves. This process can vary slightly from model to model, so follow your manufacturer's instructions. Inexpensive pumps often do not contain a gauge showing how much pressure you have in your tube. Spend a couple extra dollars and buy a pump with a built-in gauge. Knowing how much PSI you have in your tires avoids under- and over-inflation and the associated hassles.