At minimum, take your employer's match. It's free money. Beyond that, it's a judgement call in terms of current and future financial scenarios.
If you're younger, go with a more aggresive portfolio. As you get older, you'll want to decrease your risk. When you start to near retirement, you'll want almost all of your portfolio out of risky assets, and you'll be focused on preserving your capital.
It should go without saying, but don't even touch forex.
I'm not really a fan of 401(k) accounts because there's almost no control over the holdings. Sure, you can pick some random mutual fund or company stock, but you're extremely limited in what you can do, and will never know exactly what the funds are doing with your money. This can vary by company, but most companies won't give you many options. Unfortunetly, we don't really have a choice... so if you want to take a more "active" role in your 401(k), go for it. If you don't care or don't know how, just get free advice from your financial department at work (don't pay for a financial planner unless you have complicated stuff going on).
The one thing you'll probably want to read up on is how mutual funds charge you for their services: front-end, back-end, etc. A good place to start is
Mutual fund fees and expenses - Wikipedia, the free encyclopedia -- see the external links at the bottom of that page for additional information. This is something you'll want to be aware of.
HTH