The official job site for all Federal positions are listed on USAJOBS.GOV
However, there are other sites some agencies use as well, but usually they are always announced on USAJOBS.GOV website.
Also, one thing different about applying for a Federal position versus private sector, is with resume. If you are applying for a Federal position, you need to elaborate on every duty you've had in each position and I mean every little thing. In the private sector, they want you to keep your resume down to 2-pages, at the most; however, when applying for Federal positions, they like approximately 1-page for each position you're held. If you have any questions, please feel free to email me and I'll do my best to answer your questions. Good luck!