IMO, as an ameteur investor, you should be sure you know something about a company, including its products and its market before you consider investing in it.
I'm not talking about insider info or anything like that, but you should have some reason to believe that a stock is going up. I'll give you two examples.
UPS - I've liked them for quite a while (
http://appdev.info/index.php?name=News&file=article&sid=26). I like the company, and there's no denying that all the ebay trash being bought and sold has got to get shipped around the country somehow. Same goes for amazon, newegg, etc. So I watched them for a while, and at one point earlier this year, they had a bad week. High gas prices, some bad news, etc. Anyway - stock went down. I bought at $70 in early Sept, and it's now at $78, up 14%.
BA - In this case, I jumped on weakness in Boeing's competitor, Airbus. I'd been hearing all sorts of news about Airbus not being able to get their act together on their new A380, and again, BA had an off day. I scooped some up at $79, and it's $89 now, up 12.5% since late October.
Despite these successes (both of the yeilds above are for just a couple months), I've got 85% of my $$$ in mutual funds, and 15% in stocks I've picked. There's a reason why the professional fund managers are professional fund managers and I'm in software....