Many bike pumps have an adjustable nozzle that can accomodate either a Presta or a Schrader valve. That way, you can fill up any type of bike tube.
Start with the pump in Presta valve mode. Presta valves are the skinnier of the two valve types.
Unscrew the top cap of the nozzle.
Tap the pump gently so that a flexible rubber tube and a hard plastic tube come out of the nozzle.
Flip the plastic piece upside-down so that the pointed end faces out of the nozzle. Then slide this piece back into the nozzle opening.
Flip the rubber piece so that the end with the larger donut hole faces up, and slide it into the nozzle on top of the plastic piece.
Screw the top cap back on and pump up a Schrader valve.
Follow the above steps backward to change the pump from Schrader to Presta.