What is the smallest amount of Ethereum I can buy?

I'm a beginner in the world of cryptocurrency and I'm interested in Ethereum. I've heard a lot about it and I'm considering making my first purchase. However, I'm not sure how much to start with. Can you tell me what is the smallest amount of Ethereum I can buy? I don't want to invest too much at first, just to get a feel of how it works. Also, could you recommend a reliable exchange or platform where I can buy Ethereum? I'm a bit confused about all the options available. Thank you for your help!