To get admitted into M.Sc. CS course of DU, you have to clear both entrance exam and interview. The weightage of the entrance exam is 85% while that of interview is 15%. The entrance exam consists of 10 questions from English, 15 from Mathematics and 25 from Computer Science. So, clearly you can score good if you have good command on basic maths and English. Plus, since you have cleared your B.Sc in CS, thus you can answer CS questions also. Go through the important topics of your 1st, 2nd and 3rd year. Just, revise them. You don't need to learn anything new. The exam is not that tough and you will easily clear it if you have basic aptitude.