Beginneri need help please

Hello every body my name is shadi N almjdlani from syria and i 'm student at IT University .
I have smal project , i want to control the led that on arduino uno board by LDR and Application C# from PC in the same time .
My Idea i thing i can change the control between LDR and Application By Button on app .
I want tips and guidance thx a lot 8)

Post the code you have so far... do you have 2 different sketches, one that works with the ldr and one that reads serial? You could combine them something like this pseudo-code?

if button is pressed
    read ldr
    adjust led according to the ldr value
else
   read a character from serial
   adjust led according to the character received