The topics to read :
read those topics and start solving questions from gate papers , after that verify your answer with the best answers given in GO ,
to answer randomly questions http://gateoverflow.in/activity click here and provide answers .
For different subjects you can give answers here http://gateoverflow.in/categories
For Free tests --> http://gateoverflow.in/exams
There you will find subject based tests along with Full length Mock papers, environment is same like GATE.
Step 0: Study the topic/subject. Use Bikram Sir's link above.
Step 1: get a hold of subject-wise and topic-wise unsolved questions. You may use gateoverflow book (only questions). https://docs.google.com/file/d/0B8_aYGBndW4HT1RCZFFPSmg2S1k/view
Step 2: try to solve it yourself
Step 3: use the question link to go to solution or type first three words of question into google if youre not using GO book. Verify your solution with the given solution and reasoning as well.
Step 4: Make sure you have a clear idea why the remaining three options are incorrect. It is a very important part of preparation. If possible skim through the discussion to see what are the doubts of other people...the discussion of questions here on GO is really informative. Participate in discussion if possible.Post your doubts here or on the FB group.
Step 5: Repeat the above steps at least one more time before touching any test series/institute material/reference book exercises.
ps: dont forget IT question papers from 2005 to 2009