Hero Images/Getty A 401(k) is an employer-sponsored retirement plan that makes investing simple. To invest in a 401(k), you need to make three decisions: how much of your salary you want to contribute, which funds you want to invest in, and what percentage of your contributions should go toward each investment. Generally, it's best to avoid funds with expense ratios above 1%, unless your company offers a contribution match that's higher than the fee. You can change your contribution rate and manage your investments at any time through your account on the plan provider's website. Coming soon: the Personal Finance Insider email.