Light travels realy fast!You wod need an extremly acurate times to do so and an super fast way of detetcing light that bounces back.
If you have seen the devices that you point at somting and they tell the distance most have an laser.This is probobly confusing you.The laser is there just so that you cann see the distance of what object you are mesuring.The meshuring is done whith sound.The device makes an ultrasonic beep of about 40kHz (you cant hear that becose the feq. is too high) and then listens for the beeps echo.It also meshures the time taken from the making the beep and the echo to come back.
This time is then multeplyed by the half of the speed of sound and you have the distance.
The easyest way to do this is to use an microcontroler.(An microcontroler or MCU is a whole programable computer in a single chip whith evryting it needs like RAM,the CPU,Porgram and data memory,comunicatin...)
As for the stuff you need:
-PIC microcontroler (dont wory there things are cheap)
-An ultrasonic transdocer
-Op amps
The PIC microcontrolers from Microchip run up to 20Mhz.All it needs to do is first output an 40 Khz signal on one I/O pin and count the ms until it gets an pulse from the reciver that the echo has arived.Then it makes a litle math and displays it on an LED or LCD display.Then is repets this proces and keeps doing it until you shut it off.
It not all the complicated as it seems