Proceed with caution...
Other than just printing off the applicable blank forms acquired on line, I would recommend completing and handling any and all other passport application and / or renewal activities either by mail or in person at the U.S. Post Office or in person at your city / town hall --- rather than actually submitting any personal data online.
The recent pervasive "breach" of OPM (the U.S. Government personnel system --- yes, I know, it's distinctly different from and unrelated to the State Dept.) databases has justifiably eroded confidence in the the U.S. Government's attention to cyber-security and the adequate protection of potentially sensitive data in their possession.
I personally know a fair number of Federal Criminal Investigators (a.k.a. Special Agents) of both active and retired status with Secret (or higher) security clearances for many years, whose entire personal history, family info, references, financial investigation results and files (even their fingerprints, in some instances) have now been completely compromised. If the U.S. Government can't and / or won't adequately protect the records of its' own Criminal Investigators who all require high level security clearances and periodic renewal thereof, how much effort do you really think actually goes into protecting the personal data of Citizen Joe Schmoe? :annoyed: