Hi Jeff,
First you would want to request for 501st status using this link: http://www.whitearmor.net/forum/topic/16-501st-trooper-status-requests-include-link-to-your-501st-profile-in-your-request/page-1
Then apply for EIB and then Centurion after EIB approval
Link for EIB: http://www.whitearmor.net/forum/forum/95-expert-infantryman-program/
Link for Centurion: http://www.whitearmor.net/forum/forum/66-fisd-centurion-program/
HOW TOs (a more detailed guide for EIB and Cent respectively): http://www.whitearmor.net/forum/tutorials/article/36-howto-expert-infantry-application-guidelines/
& http://www.whitearmor.net/forum/tutorials/article/35-howto-legion-centurion-application-guidelines/
The links can be found in the forum homepage itself so you won't have to keep referring to this post XD