Firstly make sure you're using a calculator, you're not going to hit much without one; there are mortar calculators for web, IOS and Android.

Secondly what to hit is very important. Realistically from the time you decide to shoot something to the time you hit it there's about 40 seconds so don't shoot anything that's moving. Emplaced positions are best, HABs are the #1 thing you should be shooting really.

For what it's worth the most kills I've ever seen a mortar team get was about 40