The clutch will not pull in for a number of reasons; refrigerant charge is too low, bad cycling switch, bad clutch, wiring short, etc. so don't use that as a diagnostic.
If you really want to tackle this yourself, I would invest in a set of A/C gauges. You can get them pretty cheap at Harbor Freight. Check the pressures on the high and low side. If you have the factory shop manual it will tell you what the readings should be on both sides. I'm sure someone here can look it up for you if you lack the manuals.
You can add R134a yourself at home, but I recommend against using anything that has a sealant or stop leak in it. This can clog the orifice tube and you'll have a bigger job on your hands.
If you have a leak, it should be corrected properly. Many times it is a simple as an o-ring replacement, but it could be a leaking hose, reciever/dryer, condenser or whatever. In your shoes, I would probably just charge the system if needed and see how long it lasts.
My '97 has a slight leak and ends up low over the winter. One recharge with a $6 can of R134a and I'm good until next the next spring. Given the age of the car, that makes a lot more sense to me than hundreds of dollars spent tracking down the leak. If your leak is fast, you may have no choice but to fix it. Take it to a shop for that kind of work if you don't know what you're doing.
A shop will put UV dye in the system to pinpoint the leak. They will replace the component and then pull a vacuum to get all air out of the system. They will then fill the system with the proper amount of oil and refrigerant and test. It will be expensive.
