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If you have a 401(k) or 403(b) through your employer, your employer should be partnered with an investment firm to manage it (e.g. T Rowe Price, Prudential, or Transamerica). You need to figure out which company you've got and log in to your account. Ask your HR dept if you can't figure it out.

That firm will automatically choose where to invest your money unless you log in to your account on their website and tell them where you want it invested.

Most investment firms will offer a limited selection of mutual funds with a variety of objectives. They usually link to each prospectus right there on the site, and the prospectus often has a pie chart telling you where a fund's investments are located (US, Europe, Asia, etc). It will also list their performance over time, expense ratios, and other useful info, like whether they invest in large vs small cap businesses and their largest individual holdings.

You want to change both where your current investments are allocated and where your future contributions will be allocated.

You also want to try to find funds with low expense ratios (I try to stay below 0.10% unless it's a fund I really like and am willing to make an exception for). Anything titled "index fund" is likely to be low. Your money is almost guaranteed to be automatically invested in funds with high expense ratios, cutting into your long-term growth, because the investment firm makes big bucks giving your money to people who aren't wise with it.

If you want to get serious, you can even set up a personal choice account where you can totally independently decide where to invest your money, even in individual stocks. This comes with significant risk and is not a great idea for laypeople like you or I.