Once you turn 21 years old, and assuming you are a US citizen, you can file I-130 petition for your parents. If they are physically inside US and if they entered the US with a visa [since expired], you should concurrently file their I-485 green card applications and I-765 Work permits all at the same time. Remember that as petitioner, you must also be able to file I-864 Affidavit of support attesting tot he fact that you are able to support them to the extent DHS may think they'd end up on welfare. If you do not have enough money, you can ask a family friend or relative [being a US citizen or green card holder] to act as a co-sponsor to join you in the affidavit. Note that there are complexities when you are dealing with CIS. As such, you will find that it is often wise to speak to an immigration lawyer before you file any paperwork with CIS.
Answered on Mar 25th, 2013 at 8:44 PM