You should focus on... everything.
Don't leave anything to chance. You need to know how everything will work. Anything that can go wrong, will.
There isn't really any answer one can give here, unless you're asking about a workflow?
In which case, that changes project to project.
If it were me, I'd want the script to be pretty close to locked in. I'd make sure I had all my locations locked in, and that everything planned for those locations could work. Then I'd start finding cast and crew. Personally, I like to have a fair bit of time with all personal so we get to know eachother. Do read-throughs with the actors to see if any small things need to be tweaked. Simultaneously, you're talking with your DoP, sound folk, set designers, etc. Figuring out how exactly you'll shoot it. Sound folk should go through with you, so they can voice any concerns they have about how sound will be recorded (and processed in post) - maybe they'll have suggestions to make things a little more interesting from a sound design perspective (though ideally, you'd have talked to sound people in the script development stage, so you'd have that incorporated already). It all kind of happens at once, for me at least. In an ideal world, when everyone gets on set, they should know exactly what they're doing, what order things will happen in, how long it'll take, etc. Because they'll be so well prepared.
