I will help you to learn
The first exercise is to verify that your programming enviroment is working. Start the IDE, read in the example Blink sketch, modify it so the LED blinks in a heart rythm - short on, short off, slightly longer on, long off.
The second is to verify that your sensor hookup is good. You probably need a resistor in series with the emitter, to limit the current. I do not know exactly what kind of sensor you have, but probably you put a resistor between sensor and 5V and the other sensor leg to ground. Check polarity. The joint beween the resistor and sensor goes to an analog input. Load the example AnalogInOutSerial and thus you can read the sensors reaction in various positions and cars blocking.
You may find that you can hook it to a digital input pin instead, it depends on the sensor and your choice of the resistor. In which case your loop simply copies the read value to the LED value.
Lastly - the actual problem. I outlined the general program in previous post. Here I just give the central piece - you need to fill in setup and some other stuff - pretty printout, choose pin, trigger values (if not using digitalRead) and you may need to change < to > depending if the values is high or low when the beam is clear. The TrackLength should be a float. You choice of units determines if the result is cm/s or km/hour
while ( analogRead(StartSensor) < TriggerValue )
/* code waits here until 1st beam is interrupted */ ;
StartTime = millis() ;
while ( analogRead(EndSensor) < TriggerValue )
/* code waits here until 2nd beam is interrupted*/ ;
EndTime = millis() ;
This is very simple code. You need to chage the Serial.print to use the LCD - but first get this bit to work, then get the LCD. This way if the LCD shows “wrong” you know it is not the measurement if that showed right. Build your devices in steps, and get each step working.