The idea is to come up with an asset allocation – (bonds, equities, int’l, and cash) appropriate for your time horizon, and pick LOW COST index funds to match that asset allocation.
The brokerage houses that get the best ratings for low cost are Vanguard, Schwab, and Fidelity. Vanguard has crappy customer service, but super low expenses. Fidelity has better customer service – but still low cost. And Schwab is in the middle. (FWIW – I’m at Fidelity and Schwab – because Vanguard pissed me off when we were settling my dad’s estate.)
The more you save in fees/expenses – compounded over time – the bigger nest egg you’ll have at retirement. It really adds. up.