A 401(k) plan is only offered through an employer, which means you can't start investing in one on your own. If your employer does offer this type of retirement plan, you must sign up and figure out how much you wish to contribute. This is the amount that will be deducted from each paycheck. Be sure that this amount doesn't put you over the contribution limit set by the IRS. Your employer may also offer investment options, such as mutual funds, from which to choose. Your contributions will be divided between these funds as per your allocation instructions.