In Quicksort we devide the array in to about an half (not worst case) and we have left and right sides so it is 2T(n/2), now why in the end it is T(n)=2T(n/2)+n as we may need to go over all the array to arrange it to left and right sides?
$T(n)$ is number of comparisons done when quicksort sorts an array of length $n$.
To sort an array of length $n$, we must split it into two halves and sort the halves. To split the array, we must compare each element of the array against the pivot, which is $n$ comparisons. (Perhaps $n-1$ if the pivot is an actual array value rather than, say, an estimate of the median.) Then, we must recursively sort the two halves, which requires $T(n/2)$ comparisons each.
This last term n should, more precisely, be n-1, not n. It arises out of the fact that for splitting the array into 2 parts: one part consisting of all elements smaller than pivot and another consisting of all elements greater than or equal to pivot, we have to perform exactly n-1 (comparison) operations.