For learning piano, a piano is generally best! But if you need to go the keyboard route, a keyboard with 88 weighted keys (not “semi-weighted”) is very important.
The best keyboard companies tend to be Kawai, Roland, and Yamaha. Casios can be okay too. There are other well known keyboard makers like Korg, Kurzweil, and Nord, whose higher end keyboards can have pretty good action (action = key mechanics, feel/playability). However, these companies are known for synthesizers, so you’re paying more for the synth sounds and extra features rather than the mechanical action, which is more important for learning piano.
As keyboards go up in price, their keys often gradually improve, getting closer to the feel and control of a real piano. But again, a higher price may just mean more “features” not useful to piano students.
Yamaha P71 (AKA P45 which is a little less expensive) – A keyboard like this would generally be good for at least a couple years of lessons. If a student really takes off they might need a decent piano after a few years, or a high-end keyboard which would often be more expensive than a good used piano. This keyboard would be a decent bare minimum to consider.
Donner DEP-20 (The SE-1 is an updated version, a little more expensive) – This keyboard is a bit less than the Yamaha above, but I haven’t personally tried it so can’t fully endorse it. I’ve read some people prefer it over the Yamaha P71/P45.
Roland FP-E50 – Good step up from the first 2 in both sound and action.
Yamaha P125 – Also good step up from the first 2 in the same two areas.
Kawai ES110 – Somewhat better action than the first 4. The realism of the piano sound is good, but the speakers are not as good as the previous two (not an issue when using headphones).
Yamaha P515 – Also somewhat better action than the first 4, and much better speakers than the Kawai.
MIDI keyboard controllers
Keyboard “controllers” are silent keyboards that don’t have speakers or even their own internal sounds. They must be connected to a computer with piano software in order to be played. A benefit is that the high-end keyboard controllers tend to be less expensive, but they do require some tech savviness.
SL88 STUDIO – About on par with the best listed above.
SL88 GRAND – A good bit better still.
A decent stand is important. Some keyboards come with stands, but most do not. The absolute cheapest one I would consider is this “Hamzer” X-style stand, but they’re a bit wobbly, so it might work for the first two keyboards listed, but only just barely. Heavier X-style stands would be better, or a Z-style stand would be even better. The “table style” stands are typically the best.
Some keyboards have stands built-in, or that screw in with pedals attached. They’re often designed to look like small upright pianos. These are generally what people call “digital pianos.” They’re not necessarily any better or worse than “slab” style keyboards. They tend to look much better and cost more, but they’re also less portable.
Some keyboards come with pedals, some do not. Some come with 2 or 3 pedal units, which is great, but one pedal is all that’s needed for beginners. Avoid the “switch” pedals. Get a piano style pedal. Most of the single pedals are compatible with most keyboards, but not always! If you want to wait and get a decent pedal later, that’s fine since brand new beginners won’t need a pedal right away.
Good keyboards are expensive, but sometimes you can find a decent used one at a more reasonable price. Meanwhile there are a lot of free real acoustic pianos out there these days. Many are junk, but it’s not all that hard to find a decent free piano if you look around. Here’s a guide to finding a free or inexpensive piano.