Also what WallyM3 said.
Sketch out a layout then measure
Google this site & others for pics of their setups for inspiration & things you won't have thought of.
My current setup is always temp, as in rented accomodation, & is a whole 15" by 20" room, minus 3 display cabs, my library & 80% of stash.
I have x1 4" table for building, 1x 3" table for a/b (currently occupied with a tv) & a large dining room table 'shared' with my 3 girls, currently covered by LEGO, an ARC c/w animals & lots of 'artwork'
DON'T use folding/temp tables for building, as they will er... fold!
Principal considerations:
Desk, Lighting, ventilation, flooring & safety.
Is your desk: big enough, solid/firmly fixed,
Is your lighting: natural, adequate, got spotlights or daylight sim bulbs, avoiding flouresent striplights where poss.
Is you work area ventilated, by window or extraction, do you need a spray cabinet with filters/extraction? Are you working with noxious/toxic paints/thinners/glues.
Flooring: laminate or vinyl is better than carpets for avoiding dust & static (& the CM who shall not be named).
Safety:
Get a good office chair if possible, money spent here isn't wasted, ditto lighting & ventilation.
Because my workspace is shared with little ones I have a lockable drawer pedestal for scalpels, sharps, files, paints, thinners glues, etc, & get into the habit of locking them away.
Ditto consider pets, if you got them, make sure they can't get to your stuff or any models with drying paint. See glass fronted lockable display cabs.
Heavier equipment: If you got lathes, presses, drill stands, etc, these need to be firmly fixed, etc, & far from your painting stand.
Finally Electricals, think about what you need in terms of lighting & outlets, put them where they will not trail over tables, work areas, walkways, or anywhere where they will be cought, twisted tangled crushed or cut...
Does my H&S hat show through here...