Creating a Calculator

I am creating a calculator for a touchscreen. Won’t have a keyboard. I created a big keypad on the screen.
I want the user to be able to click on an certain input text field (value1) and then hit the numbers on the keypad to enter data into that textfield. Then click on the next input text field (value2) and put data. Then hit calculate and have a dynamic text field display the data. I’m not very familiar with focus and can’t figure out how to keep focus on an input text field while I hit those buttons. Anybody have an answer?? Thanks.