Q: I have $200 to invest in stocks or mutual funds. What’s the least expensive way to invest this lump sum?

A: When investing a few hundred dollars, you need to pay particular attention to the commissions you’re paying.

A commission is a fee your brokerage firm charges you to buy or sell a stock or other investment. Thanks to the rise of online trading, many leading brokerage firms offer very low commissions. They have dropped dramatically over the last decade to well below $10 a trade at most online brokerage firms.

The lower your commission, the better, since you’re just starting out. Consider that even taking $5 for a commission from your investment would amount to 2.6% of your portfolio’s value, which is a meaningful amount. Compare that with a $10,000 portfolio, where a $5 commission would be just 0.05% little more than a rounding error.

In your situation, you would be best served by buying a diversified exchange-traded fund, or ETF. Buying just one ETF, such as Vanguard’s new Standard & Poor’s 500 ETF (VOO). you can spread your investment across 500 stocks in 10 market sectors at a low annual cost of 0.06%. This is going to be the most cost-efficient way for you to invest in the stock market, while reducing your exposure to company-specific risk.

While many brokerages offer reasonable commissions on ETFs, and some even offer $0 commissions for select ETFs, your situation limits our search. Most importantly, you need a brokerage that has no minimum deposit, as your $200 initial deposit might be too small for many online brokerage firms.

Fidelity, Vanguard and Charles Schwab all offer free commissions on select ETFs, but they also require minimum deposits of $2,500, $3,000 and $1,000 respectively.

But don’t worry, there are brokerage firms with low commissions that have no account minimums. TradeKing and Zecco are both online brokerages that would charge about $5 per trade on an account of your size. There’s also SogoTrade, which charges $3. Another option is Sharebuilder.com, which charges a $4 commission as part of a delayed automatic investment program. Your trade wouldn’t be executed right away, but rather, queue and then conducted at a set time in the week. If you want your trade to go through right away, the commission at Sharebuilder.com is $10.

So, you see you have options. You might want to take at look at all the brokers’ websites and decide which is best for you. As you accumulate assets, you will have other options to buy ETFs, even at no commission. But these sites are a good place for you to start.

Matt Krantz is a financial markets reporter at USA TODAY and author of Investing Online for Dummies and Fundamental Analysis for Dummies. He answers a different reader question every weekday in his Ask Matt column at money.usatoday.com. To submit a question, e-mail Matt at mkrantz@usatoday.com. Follow Matt on Twitter at: twitter.com/mattkrantz