Start with the Fuse. (It will be listed on your Fuse Panel). If that seems OK, then next check the Compressor Relay. (It will be listed on your Fuse/Relay Panel). If that seems OK then - if you can - check the tightness all (if its possible to get at them) AC Hose Couplings. If they all seem tight, then check the operation of the Compressor itself by taking the top off the Compressor Relay and manually operate the Relay Contacts. (WITH SOMETHING NON-METALIC!). If the Compressor operates, then your Relay isn't getting electrical power. Next check the Presostat. This works off the AC Coolant Liquid contained in the System which operates under pressure. The Presostat detects the AC Coolant pressure (via in-built pressure sensor) and, if the pressure is OK, sends an electrical signal to operate Compressor Relay. If you get no electrical signal from the Presostat, then a likely cause is either the Presostat itself, or a blockage in the Evaporator, OR at worst, Leaking AC Hoses and/or the Specialised AC Hose Coupling Washers.
A point of note: When the technician charges the AC System with new AC Coolant (Liquid), the Charging Equipment normally checks the Required Coolant Pressure for your vehicle. Before (re) charging your Cooling System through the Compressor Valve (looks bit like a Tyre Valve), the Pressure is held for about a minute to confirm your AC System is actually holding the Pressure steady and is not 'dropping' due to a leak somewhere.
Hope this helps.