The ETF / mutual fund decision matters less than starting.
Over half the value of publicly traded companies is in the U.S. You can peek at a world fund, and it will show over 50% allocated to U.S. So starting with a total U.S. stock market investing is probably safe. it's familiar - it's every company you already know (Microsoft, Amazon, Apple, Google, Facebook, ...) and many you don't. So I'd start there.
I personally favor ETFs. A downside of mutual funds is that when you sell, you get the price at the end of the day - not now. With ETFs, you get the stock price from right now. With mutual funds, you can invest every last dollar.. you can have fractional shares - you can't with an ETF. If you buy VTI, that means whatever can't buy $150/share sits idle.. up to $149 or so. Over time that matters less.
Let's say you put $1,000 in today. One possibility is the stock market goes up 1% in 2 months. If you decide to switch investments, you would sell it at $1,010. Then you buy something else. At tax time, your $10 profit would be taxable, so you'd pay 22% (median tax bracket) or $2.20 in taxes. As an aside, if you hold investments for over a year, your tax rate drops to 15% (most people), or just $1.50 in taxes. So once you buy something, you can sell it later and take a loss or gain - the IRS shares in it, either way.
Good luck - hope you start investing.