A4kash:
That TOTALLY depends on what you do with your Arduino.Do you use it to light up LEDs? Drive motors? Build robots? Log analog values to SD cards...?
What are the necessary parts to buy in order to drive motors and build robots?
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MOSFETs, resistors, diodes, capacitors...you won't need many different types of each though.
Motor drivers tend to be high current and need lots of protection from voltage spikes. Prebuilt modules are good if you're starting out.
eg. https://solarbotics.com/catalog/motors-servos/motor-controllers/
There are stores specialized in building robots, that one's quite popular...