basicaly you have to follow a code. except that it is very difficult to design a column.
it is clearly mension on the code.
few steps
first find column axial load and bending moments in both ends
then find wether column short or slender
find the bending stress and axial stress
form the charts given in the code you can find the required reinforcement area.
My site
http://eurocode-2.blogspot.com/