I have been asked this question several times. I could give you a diplomatic answer that it depends and it could differ from person to person but in this post i will answer this question in a general sense.

IoT devices are physical hardware that can connect to the Internet and can collect, exchange data with the network. Also in some cases they can act upon that data to react to the physical world.

So here is a general list that can get you started:

1. Switch/Router Connected to the internet  (if its a wifi router make sure it can support 2G Frequency)
2. An Arduino module (For starters i would go for Ardunio UNO)
3. A Basic Starter kit containing (LEDs, Jumper wires, Breadboard, maybe some sensors)

I will be publishing some youtube videos soon where i would walk through the basics but feel free to search youtube for other tutorials to get you started.