VT, or Vanguard Total World, is probably the simplest, most diverse equity investment you can make. It contains thousands of stocks from all over the world, and you own a proportional slice of all of them. So I'd second that "VT" recommendation.
You might want to view the expense ratio before you invest - that's a good policy in general. For "VT", the expense ratio is 0.09%. That includes half U.S., and half international stocks.
When you setup your Roth IRA, did you pick a brokerage firm? Where did you invest the money inside the Roth IRA?