Oh god, untrained auto HVAC here we go...
You need an epa 609 registration in some states.
You need to know factory specs of the system (R134a amounts and oil amounts)
Replace the dryer (about $20)
New o-rings wherever you can find them (they are the cause of 99% of your leaks) Before installing them, dip them in HVAC oil, and ensure the mounting location is clean!
Get a PROPER hvac gauge set (high side and low side pressures. The can with a gauge is junk and only reads low side in a ballpark guesstimate)
Attempt to pull a vacuum in the system. If you cannot, continue hunting for o rings. Listen to the lines and components. If you cant pull a vacuum, and run out of ideas, get r134a with leak detector and stopper, and follow their directions (may seal o-ring leaks, but unlikely. Useful under UV light to trace leaks)
The HVAC hoses have standard sizes for high, and standerd sizes for low. You cannot connect them incorrectly, but still hold the can with the bottom facing away from you.
Not a bad video