Unfortunately, it's such a huge topic and people have spent lifetimes discussing the art of investing. I've been an independent investor for the past 10 years. I have a degree in business and I've also completed the Canadian Securities Course. (This doesn't mean I'm smart though. There's so much to learn out there but I can think of a few key things.)
1) Diversify - this is key. Never put all your eggs in one basket (just look at Nortel)
2) Know your risk and adjust your portfolio accordingly. riskier investments usually have a higher return. Build a portfolio that suits you and only you! you'll have to know 3 categories of investments.
a) T-Bills (almost like cash); easily liquidated; lower risk; low return
b) Bonds; higher risk and higher return than T-Bills.
c) Stocks; highest risk of all, but highest return usually.
Most portfolio usually have a percentage of all 3 depending on your age and risk tolerance level. Those closer to retirement age usually holds much less stock than someone young who can afford the risk.
3) If you are starting out, stick with mutual funds because they are diversified already, plus they allow you to invest smaller amounts.
4) when buying mutual funds, check the MER (Management Expense Ratio) on the fund, over time, your investment will suffer with a high MER. MER is how the fund manager makes a living. Lower MER means more money working for you.
4b) There's no reason why you should be buying a no No-Load fund. Buy from a competitor that offers No-Load funds.
5) I recommend index funds because it is extremely difficult to beat the market. Over time, index funds (TSE 300, Dow Jones, S&P 500) tend to perform better than most actively managed funds. Plus the MERs on index funds are much lower. Another bonus is you can simply watch TV to see how your stocks are doing!!
6) invest regularly. Timing the market is another very difficult thing to do. Also, stick with a "buy and hold" strategy. You are investing for the long term.
7) don't buy individual stocks unless you are an expert and have a lot of "fun money". whatever information you see on stock quotes (like mergers and acquisitions) are 20+ mintues delayed plus commission on trading can be expensive if you're dealing with small amounts

Arm yourself with as much knowledge as you can (who's got the time right?). The more you understand, the more you'll be able to reach your investment goal.