wsjboy

If in the US, buy index fund.

wsjboy

there are several kind of index funds, mutual funds and Exchange Traded Funds (ETFs)

Index Mutual funds: no commission but have some limitations on redemption, depends on the fund family you choose

Exchange Traded Funds (ETFs): trade just like stocks, you need to pay commission, unless your broker  offer free trading on certain group of ETFs

What you need to pay attention to for both is the fund expense ratio, if everything else is similar, then choose the low expense ratio ones.

Index fund concepts was invented by Vanguard group, their expense ratio is the lowest. They also offer zero commission on their own ETFs. Also very low commission on stock trading. If you are new in trading, Vanguard group is probably a good choice.