I think it is great to ask!
I learned about investing in a few ways and I doubt they will help you, but I'll share since there are lots of people who read these threads. First, I have a degree in finance (and marketing) and we covered investments in several classes. Mostly, though, I worked in banks and credit unions in my twenties and worked in securities for five years and talked to lots of people. Also, I took a few classes and studied for my series 6 and series 7 licenses (and two others). My knowledge all came from a career aspect, so I don't have much advice for the average person in today's investment climate in learning about investing. I can tell you there are a LOT of different philosophies on investing, so you can read a wide variety of books and talk to many people and get a lot of different advice. That's okay, in my opinion. The more you know and understand, the better you can make educated decisions.
Meanwhile, yes, you can open the account without knowing much. Read the exact offer. I believe we had to place a certain number of trades within a specified period of time in order to receive the bonus money.
Sharebuilder offers a portfolio analysis tool. Try it out. Answer the questions and see what it recommends as investments. Even with all my "knowledge", I did this and followed some of the recommendations. My training and experience from a career perspective was in a very different economic climate than now. I invested small amounts in the recommended funds and did my own research for larger amounts.
If your parents or grandparents are successful investors, ask some specific questions. I don't listen to all of my dad's advice for many reasons, but mostly you have to think about your lifestyle and where you and your family are in the natural cycle of life. Parents are in a different place than their adult children and their investing needs are different.
In general, your risk is mitigated (lessened) by diversifying -- don't put all your eggs in one basket. Have some liquid cash (easy to access, such as a local credit union), some liquid cash in harder to access accounts (usually a higher interest rate or lower fees, such as an online bank), and a variety of investments. Build slowly, so if you get burned it is minor and doesn't wipe out your savings. Mutual funds are lower risk than stocks because mutual funds have a variety of investments (a mix of stocks and bonds, usually). Stocks are investments in a single company. When it does well, great! When it does poorly, ouch!
Keep in mind the differences between insured accounts and non-insured accounts. Have most of your money in insured accounts, but don't be afraid to have money in non-insured accounts. When in doubt, read the fine print and ask questions. Deposit accounts at credit unions and banks, such as savings and checking accounts, are federally insured. Investments through these same financial institutions and other brokerages, including Sharebuilder, are not federally insured. You receive a higher rate of return BECAUSE you are accepting more risk. It is not a bad thing, but one needs to go into it with their eyes open. Fear holds many people back. Educate yourself and ask a lot of questions. I accept more risk than the average person; probably because of my formal training and exposure. The portfolio analysis will ask you about risk and returns (among other factors), so think about those things and talk to your spouse/significant other.