Shoot what works for you. There are "optimum" finger placement for triggers depending on the type of gun/trigger, but if you can't shoot well, the optimum way, then don't shoot that way. Usually, I'll use the first joint when I'm shooting revolvers. It works well for the long, heavy trigger. But on my Glock, I'm using the pad of my finger. When I used to shoot target .22 competitively, I used the very tip of my index finger.
Do what works, and don't worry about what the "right" way to do it is, if that doesn't work.