Generally, a 401K is not the greatest investment. If your employer contributes to it each year, it makes sense, if not invest on your own.
Whoever manages the fund for the employer will have a list of the mutual funds that are available. The list should show you the performance of each mutual, pick the ones with the best performance. You can pick as many as you want, as long as it adds up to 100%. meaning you could have 10 mutual funds and contribute 10% to each.

Tax wise, if your income was $20,000 and you invested $5000 in a 401K, that reduces your taxable income to $15,000. Eventually you will pay the income tax when you withdraw. But the idea is later in life your total overall income will be less, so you will pay less in taxes on the amount. With the Roth, you pay the income tax now.