If you havn't done this before you may wany to pick up a Chilton or Haynes Manual but it's really not too difficult.
1) Identify which type of Freon you have (R12 or R-134).. Usually the R-12 Freon cars have a fitting that looks like a tyre valve. The R-134 has a fitting that looks like a high pressure air or hydraulic fitting. Also look for identifying labels under the hood that will tell what type of Freon you have. Do not mix different Freons!!!
2) Identify the low pressure side of the system, Low pressure is coming from the evaporator side(under the dash) to the compressor, high side will be from the compressor to the condensing coil ( in front of the radiator).
3) do not connect to the high side unless with appropriate gauges
4) Hoping you have R-134 buy a refill kit at parts house (advance, Pep Boys, Auto Zone). R-12 now requires a technician permit to purchase unless you can find some aftermarket. The fill process is very similar.
5) With engine running and A/C on high add one 12 oz can until the A/C blows cold and the compressor stops cycling off and on.
Do not overfill, as soon as the compressor stops cycling-stop adding

6) Be very carefull to not contaminate your system with outside air, e.g. bleed some Freon out of the container into the refill line before you connect to your car system and have the can valve very slightely open when you connect and disconnect.
Good luck
Al