The L9110S can't supply a great deal of current (less than an amp per channel). I assume you have checked it can drive your stepper.
Use any GPIO you want for the L9110S inputs.
You will need to connect a Pi ground to the power supply you will use for the stepper driver. If you will be using the Pi 5V supply then just connect a 5V pin to Vcc of the L9110S.
Which Python stepper library are you planning to use?
EDITED TO ADD
As I say which GPIO you use is fairly arbitrary. You can use any you are not using for another purpose.
As it happens I have 5 (4-phase) steppers using ULN2003A as drivers set up on a Pi B+.
I am using the following GPIO
s1: 17, 15, 14, 4
s2: 18, 27, 22, 23
s3: 25, 9, 10, 24
s4: 21, 20, 26, 16
s5: 6, 12, 13, 19
I chose those as they are grouped together on the expansion header.
Connect your 12V+ to Vcc, 12V- to Ground, and also connect the 12V- (power supply ground) to a Pi ground.
Connect your chosen 4 GPIO to the A1, A2, B1, B2 pins of the L9110S.
I don't think there is a standard Python stepper library. Google will find lots of examples. Choose one at random and if you get stuck post a new question.