How can i convert this to vba code

Hello,

How can i convert the below statements to vba code?

I just want:

if Checkbox1 is check and B2 is empty or equal to zero then
B2 = Label1

If Checkbox1 is uncheck and B2 is empty or equal to zero then do nothing

If B2 is not empty or has a data on it, then a msgbox will prompt "Data already exists".

Appreciate your prompt reply.