I am planning to do a robot controlled by voice. i just don't know how to make it or where should i start. I'm beginning to like the world of robotics. Please help me..
I hope you realize this is a very open ended plea for assistance.
First you need to decide which is more important to you at the moment: A moving robot, or voice control. Both can potentially be huge undertakings.
Then you got to think about what you want to do: Do you want to build from a kit, or do-it-yourself all the way, or some combination of both?
Do you have the tools, education, and skills to do any of this? Have you done your research on what it will take to build a robot, as well as add voice recognition for control?
How big do you expect the robot to be? Something small (a desktop "rover")? Something larger (R2D2 sized - a "lab" bot)? Something even bigger (refrigerator to car sized)? Something gargantuan (Gundam sized)? The decision will be determined by your needs, skills, shop (and storage) space, tool availability, and most of all - budget (ie, how much you plan or can spend to build it).
If you are just starting out, I would suggest sticking to a desktop rover, built from a kit. Many options are available there; ideally the kit should use continuous rotation servos for wheel control (simpler to interface to the Arduino this way); the other option is one that uses DC motors plugged into an h-bridge (as a shield, usually - L293 or L298 are common h-bridges used on these boards), which is then controlled by the Arduino. This is more complicated to put together and control, but not by much.
You'll also want some sensors; to start off with, some simple Sharp IR distance sensor(s), and/or some kind of ultra-sonic distance sensor(s), would be useful. Depending on what you want to do or how you want to do it, you might want to mount one of these sensors to a servo to "pan" it around (like a radar unit) - or you can use multiple sensors if wanted.