I am in 6th semester of B.E. CSE. How should I start preparing for the campus placement?

One very vital thing is the be very confident in the fundamentals of your core subjects. So, keep themselves brushed up- atleast 3–4 subjects should be quite thoroughly done.
I hope you have been working on your communication skills.
You can also start practising some aptitude questions that you get all over the internet.
And Finally, think of your c.v, find the possible holes in it and try to rectify them. You still have time.