Q:

How to calculate time complexity for this algorithm

Compute the complexity of the following Algorithm.
i = 1;
while(i < n+1)
{
   j=1
   while(j < n+1) 
   {
      j = j*2
   }
   i++
}

A:

Ask yourself, in which way does i increment grow towards the final value n? How many times will the outer loop run for a given n?
The same for the inner loop. I recommend you read through somthing like this or this SO post and maybe start with some examples:
n = 100;
i = 1;
while (i < n+1){
    j = 1;
    while (j < n+1) {
        j = j*2
    }
    i = i+1;
}

How many times exactly will both of the loops run?

