Not everyone can contribute to a Roth IRA：

First you must have employment compensation and then there are income limits.
In 2012, the limits for single filers is income up to $110,000 for a full contribution; $110,000 to $125,000 for partial contributions. For joint filers the 2012 limit is $173,000 for a full contribution; $173,000 to $183,000 for a partial contribution.

If your income is over these limits, the only way you can take advantage of a Roth IRA’s tax-free withdrawals is by converting money from an existing retirement account such as an IRA.