On individual stocks, I would avoid them. Most people, including the financial and internet "experts", lose to the indexes in the long-run. So to answer your question: an index fund should be the core of just about anyone's portfolio.
Since you have a long time horizon, you're almost certainly going to do the best with a total market index fund or ETF. VTSMX or VTI would be two good options but you can find similar funds at Fidelity and Schwab. When you buy this, you own a small part of just about every company in America, at an extremely low cost. In short, it's a great time to be an investor!
Also, Roboadvisors (like Betterment) and Target Date funds are not the worst things for people who do not know what they are doing and do not want to learn, but they will charge you a slightly higher fee. It usually costs around 30 basis points (.30 percent a year) compared to the 3 basis points (.03 percent) you spend on something like VTI. A roboadvisor might be a better option for you if you do not want to learn about asset allocation and risk tolerance as you get older and your wealth really grows. They literally do all of the work for you. Your job is just is just to fund it.
Also, consider holding these investments in tax advantaged accounts like a 401k (if your company offers a match, always take it), a Roth IRA, or a traditional IRA. Otherwise, you're paying taxes on dividends and capital gains. This, again, depends on your goals. In most instances, you should not withdraw money from those accounts until you reach the age of 59 and 1/2.
This is just my opinion, and not meant to be taken as financial advice.
Jack Bogle. Bogleheads.