heating/cooling
insulation
electrical outlets, lighting, switches
plumbing - if a bathroom is needed
safe exit in a fire (a door or window)
drywall, paint, carpet, ......

Start with your local building dept. there are things to consider like the firewall (and perhaps ceiling)applications and fireblocking most importantly. You don't want to void your insurance.
Most building depts will be extremly helpful for homeowners in these situations. Most also has hand outs that will help you in your planning, permiting & design of the space.

Yep. Start with your Building/Zoning department.

Many, if not most, communities do not permit the conversion of garages to living space for a variety of reasons.

Best time to find out if yours does not allow such conversions is now...

Thanks- I would not have considered or thought about a zoning issue. Much obliged.