Although our site is focused on beginning investing in the stock market, it would be remiss of us not to mention that as a wise investor you should never put all your eggs in one basket. Indeed there is a large body of evidence to prove that investing in more than one asset class (diversification) will improve your returns whilst lowering your risk. To maximize your returns over the long run, stocks should form the largest element in your portfolio, but with the huge growth in ETF's you are not restricted to stocks from your home market - you can invest anywhere in the world in order to diversify your portfolio. Other examples of asset classes for consideration would be bonds, real estate, metals, and soft commodities.
We cannot hope to do justice to such a huge and important subject in one page, so I would refer you to our preferred book on the subject - The Ivy Portfolio, by Faber and Richardson. We also like The Perfect Portfolio, by Leland B. Hevner.