If you're appearing for GRE this year only(although it's very late now) you should do self study and practice more & more mock tests. Get a good grasp of reading quickly. The reading section is one of the most hectic and you should do as much practice as possible. Pick up their official study guides if you still haven't
wow i do not know anything that is online and based in Chennai.Some good online coaching sites are Magooshdotcom, GREedgedotcom etc..I find it esier to search for centres locally, as You need someone to constantly analyse and grade your weiting skills for the Analytical section.