Android/IOS First Person Shooter

Hey guys, I'm currently developing an fps for a mobile phone. The controls aren't that good and I was wondering if anyone knew a way to do the following: I would like to be able to control where the character looks on the right HALF of the screen by moving and dragging my finger. You can sorta do this with mouse look but it always resets and it's on the whole screen. I only want the right half to be used. Thank you so so much if you can help :D