Hopefully you've gotten started and have a nice portfolio going already. At least you understand one of the most important rules of investing - to get started early and to invest regularly in order to make the best of the markets' ups and downs.
I've been investing for 20+ years - in pretty much all types of investments available to the average guy - and by now I've wizened up about my own investing skills and invest almost exclusively in broad based mutual funds. I've set up automatic investment plans to a money market fund and regularly "sweep" the money into stock and bond mutual funds to keep my target asset allocation on track. Occasionally I also exchange money between stock and/or bond funds, but only to rebalance, and always after first considering any tax implications.
I can't say I made 500,000,000% return on this scheme, but I was able to go through the last decade without losing sleep once - actually enjoying periods of rock bottom stock prices that makes "rebalancing" so much more fun! ;-)
Personally I prefer mutual funds over ETFs because they allow me to buy in small increments (down to $50) without paying any fees. Costs are also very low if you stick to the more pedestrian funds from Vanguard, T Rowe Price and Fidelity, and generally I see no reason to venture into more specialized funds with higher costs. T Rowe Price allows you to start off with a $0 balance and build your portfolio $50 at a time.
I've recently started a web-site dedicated to the kind of investing strategies I think makes sense for the average investor. Take a look if you like and let me know what you think: www.InvestingIsFun.com
. The usual disclaimers apply: Information is provided only for educational purposes and I make no guarantees about correctness of the information or online tools provided.