Must I insert my fingers in the opening when I finger a girl?

You are more likely to succeed getting the girl excited if you stimulate her clitoris and inner lips. The vagina itself is not very sensitive and inserting fingers or a penis is normally insufficient to get the girl hot. Don’t start stroking her vulva right away, but build the tension by kissing her and touching her breasts first. Make her long for your hand between her legs, increasing the effect once you touch her there. Some girls like penetration once they are excited, but others don’t like this at all, some finding it even painful. So never insert fingers without asking. If you don’t want to ask you can insert the top of one finger carefully when she is very excited and watch her body response closely to see if she likes it or not.