How should a girl react when someone is fingering her?
Umm naturally...if you feel nothing, then do nothing. If you're asking this question then you probably are the quiet type and just lie there and enjoy or don't feel anything from it. Sometimes letting out noises will turn yourself on and moving your hips to where it feels great is a turn-on to him.
Make the appropriate noises of pleasure - acts as communication to show him whats nice and whats REALLY nice and whats OMG DONT STOP...
if it feels good you will know what to do! and if it doesnt dont do anything lol