If your car's air conditioner isn't blowing as cold as it used to, it could be a leaky compressor. If the oil has leaked out of it, it's time to replace the compressor with a new one.
To do the job yourself, you'll need to flush and evacuate the system, add oil and refrigerant and install a new drier, O-rings, expansion device and compressor pressure switch.
First, flush the system, using an AC flush solvent. Then check for impurities by running what comes out the flush through a coffee filter. If you see debris, flush it again. If you still see impurities, replace the condenser.
Add AC oil, recommended by your owner's manual. If your new compressor comes with oil in it, flush it out and add your own. Then add the refrigerant, following directions, also specifically for your vehicle. Next, install the new drier, which filters out debris in the system. Now, install the new O-rings, which are critical in preventing leaks in the AC system, making note of exactly where the old ones were located before installing the new ones.
Your new expansion device is either a valve, block or orifice tube. If your AC has a tube, always replace it. If it has a block or valve, they may only need to be cleaned.
Finally, install a new AC compressor switch, doing the following: Drain the refrigerant in the compressor, then remove the switch and replace it. Remove condensation, add new refrigerant, then turn off the engine for a few minutes and start it up again, running for five minutes. Turn AC on maximum and listen for the switch to turn on.
If the project looks like more work than you'd care to tackle, take your vehicle to a trusted mechanic who can install your new AC compressor for you.