I have a 2000 grand prix that will not catch gear.Drive, Reverse,anything.I got it tested and code PO452 "Evaporative Emission System Pressure sensor/switch Low Input"came up.What is this and where is it on my car?

Evap system fault codes are likely unrelated to trans issues.
Two separate problems seem to exist here.
First, have you verified trans fluid level and condition?. engine and trans warmed up and trans fluid at or near the full mark?
Secondly, the P0452 is for the fuel tank pressure reading too low. You may have a software update required or the sensor may well have failed. You best bet is actually the dealer. They can install software updates for calibrations and diagnose if it is a problem with the sensor or if a simple software update is needed.
Again, ask for their credentials and ask what is their diagnostic warranty is. They should stand behind their work.

The fluid in the trans is good.I have been told to change the trans filter,and that it could also be the trans shifting silenoid.If I change the trans I have also been told that it has to be ajusted.Is this true?Help!!

If you simply change the trans fluid and filter, there is likely going to be little change.
As for the shift solenoid, who is telling you to change the shift solenoid?
There are multiple solenoids inside your transmission, two of which are shift solenoids, and without thorough diagnosis, how can they be certain of your problem?
And, AND, none require adjustments following replacement.
Replacing those isn't a DIY project unless you already know what the inside of a transmission looks like.
You need to look for a genuine trans shop to properly diag your particular concern.

(Saving that EVAP code diag for later, I hope?)