We strongly recommend that people start with the basics of Arduino, before tackling a moderately advanced project like this. There are plenty of examples included with the Arduino IDE (the software development package) and many, many more on line.
You should learn to read a pushbutton, blink an LED without using the delay() function, and time a pulse before starting with the flow meter.
If you want to pay someone to design your circuit and write code, ask a moderator to move this thread to the Jobs and Paid Consultancy forum section.
Note that the LED input on the flowmeter requires a current limiting resistor.