What steps do I have to take to buy/sell bitcoins? What's the minimum amount of bitcoins that I can buy, and how much will I pay for transaction in a buy/sell operation?
For a start and to get to know the different exchanges and wallets you should checkout: https://www.bestbitcoinexchange.io/ (I'm not affiliated with that website).
Just sign up and add your bank account to purchase Bitcoin. Gemini allows you to buy/sell Ethereum as well.