AC clutch check ?

Hi there , My ac not working at all ( 97 Venture ) after spent few day of reading at this forum , i check my compressor is running but the clutch never clitch on , after i check relay , fuse are ok , but when i used jumper wire by pass at AC relay the clutch is engage .Does it mean the ac system is still working , but the R134a are low & i need recharge ?
my question is :
what will i do next ? , go to buy a ac charging can with die in it
any one please tell me step by step how to do it . I like to do this job by myself to save money. Thank you for your help .

you can buy the smallest can of freon just for testing, if it does work then it could be just low on freon.