That's not gonna be easy. You have moisture, mold and insects
to contend with, none of which are friendly to tracks and turnouts.
The best you can do is get a tarp that you can tightly tie together
under your layout with as good a 'seal' as possible. There are
drying agents available to use when the layout is covered.
I wouldn't leave any locomotive or controller outside when not in use.
You would want to wipe down your tracks using 91% alcohol
on a sink type scrubber each time before you run any trains.