[twblalock]

Read through the Bogleheads wiki, especially the pages about "lazy portfolios." There are some recommended books on that site as well.

I disagree with some of the Boglehead stuff, but the wiki is a good resource.

You don't need to do any trial and error. You just need to pick some funds and hold onto them for a long time. The funds Betterment has already picked for you are probably pretty good.