A Roth IRA is simply a tax-sheltered account that you deposit funds into, and then invest however you choose (within the limits of the firm you deposit the funds with).
For example, you could open a Roth IRA account with Vanguard. You could then invest the $3000 by purchasing shares of VOO, which tracks the S&P 500 index and has a very low expense ratio (0.04 as of last time I checked). Fidelity has a similar option, or Schwab, or whatever brokerage firm you prefer.
IRAs are basically just normal investment accounts, except they don't owe taxes until you withdraw them (and Roth don't even owe them then, though you paid taxes on the funds you deposit). They have some limitations regarding options trading and such, but if you're a novice investor just looking to do basic investments, you'll not notice.
Then, your IRA would go up or down in value as the market went up or down in value.
You do have some restrictions on when you can withdraw the funds; Roth IRA has fewer than a normal IRA, as you can withdraw the capital (the amount you deposited) without penalty, but the profits cannot be withdrawn until you're retirement age (I won't put an actual year, as I suspect that actual year will change by the time you're that old; but think 60s).
The reason not to invest in an IRA is if you plan on using the money in the near future - even as an "emergency fund". You should have some money that is not invested aggressively, that is in something very safe and very accessible, for your emergency fund; and if you plan to buy a house or whatever with the funds, don't start an IRA. But if this is truly money you want to save for retirement, that's the best place to start.
**Note, this is not investment advice, and you should do your own homework prior to making any investment. You can lose some or all of the value of your account while investing.