Check the pressures once a month and before a long trip, with the tires cold. The right numbers are on a sticker inside the driver’s door frame or in the handbook, not on the tire itself.
Use a gauge from any auto parts store and add air at a fuel station until each tire matches. Do not forget the spare if your car has one.
If one tire keeps losing air, bring it in: a puncture repair is quicker and cheaper than a new tire (sample).