A few weeks after USCIS receives your completed application, it will send you a receipt notice called I-797C, Notice of Action. It is a very important document. It extends your residency for another year while the application is pending. It is your only proof of legal status until the application gets approved and can be used for employment, travel and other purposes. You must also carry your expired "Green Card" at the same time as it has your photo on it. Following submission of your application, USCIS will notify you when to go to an Application Support Center to be fingerprinted.
You can use the Application Receipt number (can be found on any of the notices that you would have received from the USCIS) to check your case status online.