Why does my 2001 Infiniti G20 make a squealing noise whenever i start it and turn the AC on?

It sounds like you may have a loose or dry belt(s). You can get some belt conditioner that you spray on the belt while the engine is running, or you may have to replace the belt(s).

Napa and Pep Boys should both carry some.

Replace the belt. Belt “conditioner” is a useless stop gap. All it does is cover up the problem then one day you’re driving along and “bang” your belt flies off and your power steering goes meaning you have to strong-arm the car everywhere. Not bad if you’re used to a vehicle with stiffer-than-manual steering, but if you’re not prepared for it and you’re moving at a good speed, it’s downright dangerous. Get that belt replaced ASAP, it’s a $40 serpentine belt, takes a good mechanic about ten minutes to do.

The thing is i’ve replaced the belt many times but the problem still persists

Hmmm… Should have mentioned that.
Anyways my statement was moot as I just remembered your G20 uses the SR20DE which has two belts, one for Power Steering and Water Pump, and a longer one for A/C and Alternator. Which means that either the alternator adjuster bolt is lacking thread-locker causing it to loosen, or your alternator pulley bearing is wearing out, or your alternator is worn out, or your A/C compressor clutch and pulley assembly is wearing out. I would bank on it being the alternator as that is the usually the first thing to go when your car hits the 10 year mark.
The pulley itself is easy to change, however most of the time it’s worth it just to replace the whole unit. Just pray it’s not your A/C compressor clutch assembly as those are a pain in the ass to replace, not to mention expensive. However and I can’t stress this enough never take an Infiniti to a joe-blow-mechanic or you will be replacing everything until he finally gets it right. Take it to a Dealer, either a Nissan or Infiniti as they can get you the parts and service you need.

I once had a joe-blow mechanic (Shell station) adjust my water pump belt. He adjusted it tight as a piano string. The extra tension wore out the bearings in the water pump and popped the seals. The engine ended up shooting coolant all over Hwy 91, while I was driving at highway speeds. So go to the dealer and have them fix it right.

It’s probably the belt alignment. All the pulleys need to be lined up, and if one is out of kilter, this could happen. I suppose it could also be the actual AC unit’s pulley, if you lost a bearing.

This is the exact thing going on with my 2001 g20 im Trying to fix myself! Did you end up pinpointing the problem?