How to recharge A/C on Miata?

I do most of the work myself but I never had to work on A/C. The
problem is it blows hot air and the compressor doesn't kick in
(possibly due to low level of refrigerant). I went to Autozone but the
clerk was clueless when I asked him what kind of refrigerant (they had
different cans of R134A) and hoses I need to buy. Has anyone here
recharged Miata's A/C and if so I would appreciate any tips and
tricks.
BTW, the car in question is a 10AE Miata with 32K mi.

TIA!