Hey I am a newbie and I was wondering about buying and selling a currency. I dont get what is going on and am having a hard time wrapping my head around this. When you have a currency pair (say EUR/USD), what happens when you sell it?
This is probably very basic stuff but I really dont know anything about this.
When you buy EURUSD, you are borrowing (if you use leverage) USD and using the borrowed USD to buy EUR.
Then the price changes and you use the EUR to buy back the USD and pay back the loan. Hopefully, the EUR has gone up in price so you can buy back more USD than you borrowed, pay back the loan and have some profits to keep. 
Simple really. 
Edit:
You don’t really need to know all this behind the scenes stuff, but it interests me too. If you buy and it goes up you gain, if you buy and it goes down then you lose, the opposite is true when you sell.
The words kinda get confusing since you buy into a long position, but sell out of it, or sell into a short position and buy out of it. It’s easier to say “go long” or “go short”, or “open a long trade” and “close a long trade”, “open a short position”, etc. It always remains true that you should buy low and sell high whether you’re buying or selling into or out of a position.
I would recommend that you get a free forex course and learn the basics you need to know. In order to answer your question, the first currency is called the base currency and the second one is called the quote currency. When you sell the EURUSD pair you believe that the USD will strengthen against the Euro and therefore take a short position in order to profit from your prediction. This means that you think it will take less USD in order to purchase Euros.
This difference is called spread . If you buy or sell any currency pair it goes to loss of 2 to 3 pips. This is the commission of the broker as he offered at any currency pair.when market goes to your favor then after 2 or 3 pips your profit starts. You have to see the spread of any pair before trading in it.