You can contribute to your employer’s 401(k) plan as soon as you’re eligible to participate under the terms of the plan. In general, a 401(k) plan can make you wait up to a year before you’re eligible to contribute. But many plans don’t have a waiting period at all, allowing you to contribute beginning with your first paycheck.
Some 401(k) plans provide for automatic enrollment once you’ve satisfied the plan’s eligibility requirements. For example, the plan might provide that you’ll be automatically enrolled at a 3% pretax contribution rate (or some other percentage) unless you elect a different deferral percentage, or choose not to participate in the plan. This is sometimes called a “negative enrollment” because you haven’t affirmatively elected to participate–instead you must affirmatively act to change or stop contributions. If you’ve been automatically enrolled in your 401(k) plan, make sure to check that your assigned contribution rate and investments are appropriate for your circumstances.