I'd put it VTI for now, and set up contributions to come out of each paycheck. By paying yourself first, in this manner, you avoid the temptation to time the market. In other words, you don't have to decide when to buy because it happens automatically. As a result, you buy more shares at lower prices, and fewer shares at higher prices (compared with the average price you pay over a year).
It will seem slow at first, but after a few years there will be a point where the annual gain in asset prices excess your contributions, and then it really becomes enjoyable to watch it grow.
If you start to get anxious about downturns in the market, then switching to a target date fund will reduce your volatility, but ideally you don't make the switch at a low price.
Kudos for starting early!