I am having a problem about this h bridge. What will I do with this forward input? Should I supply a voltage on it

 

What else would you do with it, its self explained, forward and reverse.

 

How can I turn the motor on using the circuit I uploaded

 

You apply a voltage to either input. The value and polarity of that voltage will depend on the properties of whatever those big triangular things are. Presumably they're buffers/level-shifters.

 

how can i apply sensors to forward input or reverse input like night detector or light detector?

 

You choose sensors which provide outputs appropriate, or can be conditioned to be appropriate, for the inputs of those triangular things.