Etrade.com Roth IRA Questions how to get money?

ok so I made a Roth IRA account on etrade... I put in $100.00 this today and made it enter $50.00 at the end of each month. But then I was messing around and decided to buy 2 shares of starbucks stocks. When i go to buy them it says it's going to my roth IRA acoount? I decided not to buy the stocks. BUT what I don't understand do you have to buy stocks or bonds to get money in a roth ira? I though you just put money in and it automaticly you just earn intrest or somthing.. can sombody plesee explain I just spent 3 hours trying to fig this out on google haha.. by the way im 18.. just in case you need to kno