If you’ve got a new job and are grappling with a company-provided 401K for the first time, congratulations. If you’ve got an old job but that last sentence reminded you that you’ve been meaning to set up your company-provided 401K plan, then you’re a terrible father but there’s no time for that now. This detailed explanation of what it is and how it works is a great starting point. It lays out the various investment options and how to determine your risk profile, based in part on your time frame — if you’re new to investing, look for a target-date fund, if it’s an option in your company plan. A 401K is one of the most valuable tools in a retirement portfolio — and if your employer matches, it is the most valuable — so, unless you plan on getting you kid back for all the diaper changing by making them change yours in 40 or 50 years, get on it.