JoeTapayer has good advice here. I would like to add my notes.
If they give a 50% match that means you are getting a 50% return on investment(ROI) immediately. I do not know of a way to get a better guaranteed ROI.
Next, when investing you need to determine what kind of investor you are. I would suggest you make yourself more literate in investments, as I suggest to anyone, but there are basic things you want to look for.
If your primary worry is loss of your prinicipal, go for Conservative investments. This means that you are willing to accept a reduced expected ROI in exchange for lower volatility(risk of loss of principal). This does not mean you have a 100% safe investment as the last market issues have shown, but in general you are better protected. The fidelity investments should give you some information as to volatility or if they deem the investments conservative.
Conservative investments are normally made up of trading bonds, which have the lowest ROI in general but are the most secure. You can also invest in blue chip companies, although stock is inherently riskier. It is pointed out in comments that stocks always outperform bonds in the long term, and this has been true over the last 100 years. I am just suggesting ways you can protect yourself against market downturns. When the market is doing very well bonds will not give you the return your friends are seeing.
I am just trying to give you a basic idea of what to look for when you pick your investments, nothing can replace a solid investment adviser and taking the time to educate yourself.