Betterment is a great place to get started as there are no account minimums. You can start with $.01 if that is what you have. Betterment invests your money in a diverse mix of ETFs. Investors can choose their stock/bond ratio in increments of 10%; Betterment determines the portfolio of the stocks and bonds.
The ETF costs are rock bottom. Betterment charges an additional layer of fees, but it's pretty reasonable. Betterment is higher than Vanguard, but still far lower than many mutual funds.
https://www.betterment.com/