1. Bolt pattern
The number of lug studs and the diameter of the circle they sit on. It's written like 6x139.7 (six lugs on a 139.7 mm circle, also called 6x5.5 inches). It has to match your truck exactly. There is no close enough.
Many trucks within one brand share a pattern, but not across brands and not always across generations. A wheel listed for your exact year, make and model has already been matched.
2. Center bore
The hole in the middle of the wheel that fits over the hub. It must be equal to or larger than your hub. A wheel with a larger bore is centered with a hub-centric ring, a simple plastic or aluminum sleeve that takes up the gap and keeps the wheel from vibrating.
3. Offset and width
A 20x9 wheel with a low or negative offset is the classic look, and also where most fitment problems start. Compare the new wheel's offset and width with your current wheel: outward change decides whether it pokes past the fender, inward change decides whether it reaches the suspension.
On stock suspension, staying within about half an inch of the current outer edge is usually safe. More than that may need a leveling kit, a lift or trimming.
4. Brake clearance
Big brakes need room inside the wheel. A 20-inch wheel almost always clears factory truck brakes, but spoke shape matters: some designs curve inward close to the hub and can touch the caliper even when the diameter is fine. Retailers list the vehicles a wheel clears; this is one more reason to buy from vehicle-specific fitment.
5. Tire height
With a 20-inch wheel the tire sidewall has to shrink to keep the same overall height. Keep the new size within about 3% of your current tire and the speedometer, ABS and stability control stay accurate. Go taller on purpose only if the truck is leveled or lifted for it.
6. Load rating
Check two numbers. The wheel's load rating, per wheel, has to cover your truck's axle weight rating divided by two. The tire's load index has to match or exceed the original. If you tow or haul, use LT (light truck) tires where the original was LT.
Keep your TPMS. The tire pressure sensors move to the new wheels or new sensors get programmed in. Ask for it when you order the package.
Questions
Do I need to lift my truck for 20s?
Not for 20-inch wheels with a tire of the same overall height. A lift or leveling kit is about fitting taller tires or more aggressive offsets, not the wheel diameter itself.
Will 20s ride rougher than 17s?
Somewhat, because the tire sidewall is shorter. On most trucks the difference is mild with all-season or all-terrain tires at the right pressure.
Can I keep my factory lug nuts?
Only if the new wheel uses the same seat type (conical, ball or flat) and thread. Many aftermarket wheels need different lug nuts, which usually come in a wheel and tire package.