Visit the official website of IIT Delhi (Indian Institute of Technology Delhi).
Goto individual departments and find out what PG specializations they offer. (M.Tech./M.S./etc in different fields)
Find out the PG courses with specialization of your interest. (Most important thing).
Check eligibility for the respective specialization / department. This may be available at a separate link on the website for PG-Admissions or something. Sometimes multiple GATE disciplines can apply to cross-dept. i.e. A student who has done B.Tech. in Electrical Engg (and wrote GATE-EE) can pursue M.Tech. in ComputerTechnology / Mathematics / Physics / etc depending upon the course requirements/eligibility by the concerned dept. (On a side note, as shown in the 3 idiots movie, it is also possible to attend courses outside your departments, but this comes at the cost of alot of time & efforts as each course demands alot of hardwork if one really wants to learn.)
Give all your energy for GATE preparations!
Once GATE scores are available, visit PG admissions website and apply formally (http://www.iitd.ac.in/pgadmission/)
Keep checking the PG admissions page, department page & your email for any updates on shortlisting/interviews/etc.
Follow the application process/interviews/etc and complete the admission procedure.
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile