401k is your retirement fund, you definitely want to put money in that. Basically it will be invested in stocks/bonds for you so it grows, and usually your employer will match your contributions up to a certain amount (like 5% your income usually), so I definitely recommend putting as much as you reasonably can in out of your paycheck. A normal 401k is tax free until you withdraw it when you retire, or you could do a Roth 401k which is taxed now so it’s tax free when it’s withdrawn