The pitch setting you are referring to is a calibration. For standard tuning it should always be set to 440. You would only change it if you had some reason to not tune to the standard pitch. For example, if you were playing along with a piano that was tuned slightly sharp or flat, or with tape recording where the playback speed did not match the recording speed (which would affect the playback pitch)
So basically, set it to 440 and forget about it. You should almost never need to change it.
When you play a string, the tuner should show what note is being played and whether it is sharp or flat. Adjust the tuning on the guitar so that it shows the right note for the string you are playing, and fine tune it so that it is not showing as sharp or flat.Standard tuning for a guitar is E A D G B E - going from the low (top) string to high string.