Resigning is mostly logistics, and the logistics have rules.
Notice is a custom, not a law
No federal law requires two weeks of notice, and at-will employment runs both directions. Notice is convention, and it mostly buys you a reference and an orderly exit. Be aware that an employer can accept a resignation immediately, so the day you hand notice in can become your last day. Do not give notice before you can afford that outcome, and copy anything personal off work devices beforehand, because access often ends within hours.
The final paycheck
States set deadlines for final pay, and they differ by whether you quit or were let go. Some states require payment on the last day for a dismissal, others allow until the next scheduled payday. All earned wages must be paid; an employer cannot withhold a final check over unreturned equipment in most states, though they may pursue the equipment separately.
Unused vacation
Whether accrued vacation must be paid out depends on the state and the employer's written policy. Several states treat earned vacation as wages that must be paid regardless of policy. Others allow "use it or lose it" policies if clearly communicated. Sick leave is generally not paid out. Check the handbook before your last day, because the policy in force when you resign is the one that applies.
When benefits end
Health coverage commonly runs to the end of the month you leave in, but some plans end it on the last day worked. Ask, because the answer decides whether you need continuation coverage for a gap. COBRA lets you keep the employer plan for up to 18 months at full cost plus a small administration fee, which is expensive but useful for short gaps and ongoing treatment. A marketplace plan through a special enrollment period is often cheaper, and losing job coverage is a qualifying event that opens one.
Retirement accounts
Your 401(k) contributions are always yours. The employer match is yours to the extent vested, and the vesting schedule in the plan documents decides that. After leaving you can leave the money in place (if the balance is large enough), roll it to an IRA or a new employer's plan, or cash out, which triggers tax and usually a penalty and is nearly always the worst option. Small balances can be force-rolled out by the plan, so keep your address current with the administrator.
Restrictive covenants follow you out
Whatever you signed at hiring survives your resignation. Reread it before accepting a competing offer, not after, and take particular note of non-solicitation clauses, which are more commonly enforced than non-competes and cover recruiting former colleagues as well as approaching clients. Confidentiality obligations are permanent nearly everywhere and cover material you may think of as your own work product.
The related practical rule: leave the files. Taking client lists, code, decks or documents to a new employer converts an ordinary resignation into a trade secrets problem, and companies do audit downloads and email forwarding in the weeks before a departure.
References, realistically
Most large employers have a policy of confirming dates and title only, which limits both damage and help. Individual managers give real references informally, so leaving on good terms with a person matters more than the company's policy. Ask a manager or two directly whether they would act as a reference before you leave, while goodwill is fresh and their memory of your work is current.
The exit conversation
Keep the resignation letter to three sentences: you are resigning, the effective date, thanks. Reasons go unsaid in writing. Exit interviews are optional in most companies and nothing said in one is confidential in any enforceable way, so treat candor there as a gift to the employer rather than an obligation.
The Department of Labor sets out COBRA continuation rights and the deadlines that apply once coverage ends. Final paycheck deadlines are set by each state's labor department.