Upgrading 08 f150 as a 17 soon 18 yr old

I’m am planning on fixing her up, I just got her in February in Omaha Nebraska at a auction and as discovering more she has water rust all the way up to door trim or it could’ve been higher I’m not sure. No service engine lights were on until maybe around April and it was an exhaust leak so I patched that up it went away and I just got one in June and I got a code reader says it’s my catalyst system efficiency below bank 1 I’m not sure if it means my catalytic converters are going bad I’m not sure how to check that. Any help would be nice I do way other mechanical issues too and questions.