Check if your job offers "401(k) with matching". That means to motivate you to put money in a 401(k), some employers match your money with their own. Getting that free money is the best thing you can do, if it's available.
Betterment's main idea, that they will do loss harvesting, only works when the market drops so far that your investments have a loss. So at first, it can work. But after several years of stock market gains, even a -20% drop isn't enough to wipe out the prior gains. Over time, loss harvesting no longer works for the money you've invested more than a few years. The expense, though, 0.35% is applied on all of your investments - even the ones that are no longer able to harvest a loss.
I'd favor Vanguard, Fidelity or Schwab over Betterment. But if you feel intimidated by changing choices, it's better to start at Betterment and figure that out later. The worst decision is sitting in cash, rather than investing in mutual funds. Even funds that charge 1.00% expense ratios are better than cash.