Do a google search on this group. There are lots of posts.
To recap...
In order to properly recharge your air conditioning system, you have
to evacuate it with a vacuum pump to remove everything from the system
- especially all the AIR that is in the system.
THEN you blead refrigerant into the system.
You need to ensure that the system will hold the vacuum before
recharging - this proves there are no leaks (well no major ones at
least).
And, of course, all of this must be done with the correct equipment
and with the local (EPA??) Environmental Standards applied! You cannot
just vent refrigerant into the atmosphere.
In English....
This is NOT a do-it-yourself project. Go to a certified A/C shop and
let them do what they do best.
If you try to save a few bucks with a $10 can of refridgerant from
Auto-Zone or Canadian Tire, you will likely cause the rapid demise of
your entire A/C system - then is will REALLY cost you!
YMMV