A frequent question we get here at Govitwiki is if civilians can obtain a security clearance. Experts project that a security clearance can increase your salary anywhere from $5,000 to $15,000, and in some cases, even more.
Unfortunately, you can't apply for a security clearance on your own. A security officer or other authorized employer representative must request it on your behalf. 
A security clearance is generally granted for a specific clearance level.
Confidential - The most basic security clearance. It usually requires what's known as a National Agency Check with Local Agency Check and Credit Check (NACLC), and it must be renewed every 15 years. This level of check currently is not used by the U.S. military.
Secret - This is sometimes referred to as "Ordinary Secret." It requires a few months to a year for a full investigation. In general it requires a NAC/LAC/Credit check, and it may include investigations of past residences, residences or businesses in foreign countries, relatives and more. Bankruptcy, criminal charges and unpaid bills may also be considered. It must be renewed every 10 years.
Top Secret - Also know as "TS." It includes what's known as a Single Scope Background Investigation (SSBI). Top Secret clearances generally afford one access to data that affects national security, counterterrorism/counterintelligence, or other highly sensitive data. The SSBI must be renewed every 5 years.
A form is available from the General Services Administration which can be used to apply for a security position Form #GSA1380 But, again, government employees are normally only considered for a security clearance if one is deemed necessary for your job (or potential job) and if you are nominated for consideration for the clearance.