ETF's (exchange traded funds) are probably one of your choices.
You buy them through a financial advisor/broker (such as Edward Jones, Charles Schwab) or online through a low-cost investment site like Etrade, ScottTrade, etc.
ETF's are a collection of investments with a common theme.
They are often better for the small investor than individual stocks because you are buying a small slice of a large collection of stocks.
Several firms specialize in running them such as ishares, vanguard, etc.
Because it takes almost no brains to run one, the internal costs are very low compared to mutual funds.
They have 3 or 4 letter trading symbols just like stocks.
For example, for a precious metals theme, there is SLV or GLD to invest in silver or gold.
For a theme in foreign country economies, there are ETF's like FXI (China), EWZ (Brazil), etc.
For a "commodities" theme, try XLB or FCX or USL (oil).
For the S&P 500 companies, use SPY or IVW.
To learn all about one, you can look up the 3-letter symbol on Yahoo Finance.
For example, here is a link for USL: http://finance.yahoo.com/q?s=USL
You can see price charts for the last 6 month or year, you can read the profile write-up to understand what they invest in, etc. Use the blue links on the web page to navigate to different info, such as the "profile" write-up.
I currently use etrade. It typically costs $9.99 to make a purchase and $9.99 to make a sale. So, USL is trading for $42.06 at the moment, so to buy 100 shares, it will cost about $4216 ($4206 + $9.99). I recommend you buy in blocks of multiples of 100 shares if the price is under $100 per share. Obviously, if you bought 500 shares, the $9.99 cost is spread over more shares.
The tough part is setting your strategy. Do you think the US dollar is going to go bust? If yes, buy commodities or foreign economies. Do you think we're going to have more deflation and deeper recession, then stay in cash. No one really knows these answers. Read news articles and make your own opinion.
Hope this helps. YMMV.