Inspect all cables for degradation of jackets, connectors, etc. If it's been in the backyard for more than a decade, the cable jacket may literally come apart when you touch it.
Same is true for all rubber gaskets, tubes, belts, tires.
Drain the transmission fluid, engine oil and coolant and get new fluids.
Some springs may be permanently bent due to being compressed for more than a decade.
The cylinders and pistons may be rusty. A complete rebuild is required and usually only an experienced mechanic with professional tools can do this properly.
Basically you need to tear the car apart and refurbish everything, replace anything that cannot be rebuilt/refurbished. Also check for corrosion on the body. It is not impossible that some frames have rusted all the way through. Expect dead/living animals in the car body and all tubes. Rats may have bitten through all non-metal tubes.
This can easily cost more than the car's worth, however if you want to do this as a hobby project you can try.