This is the question I am asked before any other, usually by someone who has not yet played a note and is worried about choosing wrong before they begin.
So let me take the pressure off first: the instrument you already have is good enough to start. If there is a keyboard in the house, begin on it today. Do not wait for the right purchase; waiting is a far bigger risk to your playing than any instrument is.
That said, two things genuinely matter, and a great deal of what gets argued about does not.
The two things that matter
1. Weighted keys that respond to how you play them. This is the whole question, and everything else is decoration.
On a real piano, a key pressed gently gives a quiet sound and a key played with the arm's weight gives a full one. That relationship is not a feature; it is what playing the piano is. Every physical thing I teach — weight, release, evenness, tone, phrasing — exists only because the instrument answers differently depending on how the key goes down.
An unweighted keyboard, where every key gives the same volume no matter what you do, cannot teach any of it. Worse, it teaches the opposite: it trains a hand to poke, because poking works there. That habit then has to be un-learned on a real instrument, and un-learning is far slower than learning.
The words to look for are weighted, hammer action or graded hammer action. The words that mean you are being sold something else are touch-sensitive alone, and semi-weighted.
2. Enough keys that the music does not run out. More on this below, but briefly: a beginner's first months need far fewer keys than people think, and a beginner's second year needs more than a small keyboard has.
What does not matter nearly as much as the shop suggests
The number of sounds. Six hundred voices is six hundred ways not to practice. You need one good piano sound.
Built-in songs, light-up keys and learning modes. These teach you to follow a machine rather than to read. The skill you want is reading, and a light above a key is not reading.
Brand loyalty arguments. Among the established makers, at a given price, the differences at beginner level are real but small, and much smaller than the difference between weighted and unweighted.
Whether it is “a real piano”. A good digital instrument in a flat where you can play at eleven at night is worth more than an acoustic you are afraid to touch. I say this as someone who performs on concert grands.
Digital or acoustic, honestly
A digital piano is the better choice for most adult beginners. It never goes out of tune, it costs nothing to maintain, it takes headphones so the household is not consulted about your practice hours, and a decent weighted one is cheaper than a year of tuning and repair on a neglected acoustic.
An acoustic upright is the better choice if you have the space, the household, and either a good instrument already or the budget to have one chosen properly. The sound is richer, the action tells you more, and it rewards you as you improve in a way that a modest digital does not.
What I would avoid: a free acoustic piano of unknown history. “Free piano, you just have to move it” is very often a piano that needs more work than it is worth, and moving it costs real money before you find out. Have anyone's free piano looked at by a tuner before it enters your house.
How many keys
The honest answer is that it depends how long you intend to keep playing.
Your first months will live in the middle of the keyboard and could be done on a very small instrument. But the repertoire an adult actually wants — the pieces people picture when they imagine themselves playing — reaches out to both ends, and a short keyboard starts refusing to play the music before your hands refuse to.
If you are buying once and keeping it, buy the full set of 88. If you are borrowing or starting on what is in the house, start there and do not think about it again for six months. I have written about this in more detail on my instruments and books page.
Two things to buy that nobody mentions
An adjustable bench. This will do more for your playing than an extra two hundred dollars of instrument. Almost every fault I see in adult beginners — aching forearms, a thin sound, uneven fingers — begins with sitting too low, and a dining chair is nearly always too low. A bench that adjusts fixes it permanently in one purchase.
A proper stand, if the instrument does not come with legs. A keyboard on a wobbling X-stand moves when you play with any weight, and a hand that feels the instrument move learns not to use weight. Something solid, at the right height, that does not travel.
So: what to do this week
If you have any keyboard at all, start on it, today, and come back to this question in three months when you know whether the habit is taking. If you are buying: weighted hammer action, 88 keys if you can, an adjustable bench, and ignore everything printed on the front panel in large type.
Then start. The instrument matters much less than the fact that you began.