With $100/month, you're limited to mutual funds. Otherwise, you pay a per transaction fee that would be at least 7% of your investment - very, very bad deal.
I'd suggest sticking with a Vanguard Target Retirement Fund: https://personal.vanguard.com/us/fun...RetirementList
Select your age, pick that fund, start moving money into it, done.
Note that these funds come with a minimum $1,000 opening deposit. So keep the money in a savings account until you have that amount.