How do you calculate the running time using Big-O notatation?

I’m still new to Data Structure and Algorithm and therefore I would like to ease my doubts. I’m required to find the Big-O running time of myMethod():

 Static int doIt(int n) {            for 𝑖←1 π‘‘π‘œ 100 do    -> 100              for π‘˜β†1 π‘‘π‘œ 𝑛 do    -> n x 100                  𝑗←1;π‘šβ†π‘›       -> 1            while 𝑗<π‘š do       -> lg n           π‘šβ†(π‘š+𝑗)/2           -> lg n/2         end while          end for          end for          }   static int myMethod(int n) {                   𝑖 ←1                   π‘€β„Žπ‘–π‘™π‘’ (𝑖 < 𝑛) {          -> lg n                         π‘‘π‘œπΌπ‘‘(𝑛)            -> ?                          𝑖←𝑖×2             -> lg n         }             π‘Ÿπ‘’π‘‘π‘’π‘Ÿπ‘› 1;   } 

For each of the line I have inserted the answer I think would be a logical value to put e.g "-> 100". However I am not confident with the answer. These are the following questions that I would like to ask as well:

  1. Does all while loop have a value of lg n?

  2. If the doIt(n) method does not have a return statement, do we still add in part of calculation?

  3. For the line with m <- (m+j)/2, the value I have inputted is lg n /2. Is that correct or is it some other value?

  4. Whenever multiplying by 2 or dividing by 2 is it always lg 2? what if n is multiplied by 3?

Thank You