Another option, if you prefer mutual funds vs. ETF's as I do, is to temporarily use one of the Vanguard Target Retirement Funds (like Vanguard Target Retirement 2035 Fund (VTTHX), for example). They only require $1,000 to open! Save your $100 a month in a savings account until you have $1,000, then open your account at Vanguard (the Roth is a good suggestion if you're eligible). Once your balance grows to $3,000, you can transfer to many different funds to meet your goals.
Save the $1,000 outside of Vanguard as their interest rates are terrible. Check on-line banks (FDIC-insured) like CIT, for example.