Sharebuilder is an excellent idea!
You might check out motleyfool.com to start educating yourself. There are many financial magazines and websites to help you also.
I'm a big believer in DRIPS (dividend reinvestment plans), which allow you to start with small amounts, and have the dividends reinvested to buy more shares over time. It's a bit of a dollar cost averaging method and totally appropriate for me, a small timer with a long time line. I was able to buy in to some plans with as little as $50 - 100. Given the amount you plan to invest, DRIPS might be worth a look so you could diversify, even with a small starting sum.
Before Sharebuilder took it over, netstockdirect.com had listings of the plans. I'm not sure where to find one now, but you can google for it.
Do you know what you are going to buy yet, or are you just now at the "let's look into this" stage, and not sure what company, or even industry, you plan to invest in? If that's the case, do research, lots of it. I like to buy companies I believe in over the long haul (I'm a buy and hold investor, so far) and direct my research that way. evaluate your reasons for investing and goals and research towards that.
Good luck. And congratulations for deciding to seize an opportunity vs just seizing.