Does anyone know a site where you virtually can BUY and SELL bitcoins and other cryptocurrency to practice? (not speculating (spread))

I'm a week into this whole bitcoin thing so be easy on me. I wonder why you would ever sell a BTC ... For example: you buy 100 dollars worth of BTC but the price drops drastically. Why would you sell it when its that low? You KNOW it's going to go up again. You could wait a month (for example), the price will be up again. Or am i totally wrong here?