If you know nothing about investing and don't care to learn, but want to be sure that your retirement savings will be sufficient unto the day, consider adopting one of the following strategies.
For example, if you plan to retire sometime around the year 2035, you'd look for a target date fund with the year 2035 in its name.
Index funds and ETFs If you don't have access to a target date fund or prefer a bit more control over your retirement investments, then an index fund or ETF is the way to go.
Stocks versus bonds If you choose an index fund or ETF rather than a target date fund, you'll have one extra decision to make: how much you want to invest in stocks versus bonds.
Now you can simply buy one stock index fund and one bond index fund and allocate your contributions according to the above formula.
You'll also need to reallocate as your balances shift over time thanks to the performance of your investments (unless you have a target date fund; this is something your fund would do for you).
The stock market prospered during the following year, while bonds dropped in value.
If you come back and look at your investments one year later, you'd probably find that your allocations had shifted due to the performance of those stock and bond markets: you might now find that your retirement savings account has 74% of its value in stocks and 26% in bonds.
In the above example, you'd sell some of your stocks and use the funds to buy more bonds.
Since you'd probably gotten a year older during last year, you'd want to shift your balances to 69% stocks and 31% bonds.