Investing for an 18-Year-Old
At 18, you gain the legal right to open investment accounts independently, without a custodian. Your highest priority before any investment account is an emergency fund of at least one month of expenses. A Roth IRA at 18 with earned income starts compounding immediately. If parents opened a 529 plan for you, it continues through education or can transfer to a Roth IRA after a 15-year holding period under current rules.
The most important financial action at 18 is setting up an emergency fund first, then starting a Roth IRA with any earned income. Do not invest money you may need within the next two years.