Once you've decided on a digital piano, there are a few other things you'll need.
The most important thing about a piano stool is that it should enable you to sit comfortably at the correct height to play with good posture. When sitting at the piano your arms should be parallel to the floor, with your wrists slightly above the keyboard. If you are sitting at the wrong height it can cause bad playing habits as well as back or shoulder ache.
If you have decided to purchase a portable piano i.e. a piano that does not come with its own integral stand, you will need to purchase a stand to put it on. A dedicated stand is very important as it will be adjustable for height enabling you to maintain the correct posture when playing.
There are various types, the most popular and least expensive being an X frame, so called because it is in the shape of an X. These are easy to adjust for height and can be collapsed for easy storage. A double braced X stand is good for heavier keyboards. If it is really heavy, you could consider a Z stand which is designed to hold large stage pianos weighing up to 500 pounds. Some digital piano manufacturers also offer their own stands particularly for their stage pianos which tend to be a bit more stylish.
The ability to play using headphones and not disturb others is one of the main attractions of a digital piano. There are two main types of headphones - closed back, where the outer part of the headphone cups have a hard enclosure, or open back, which leaves the back of the headphone cup open.
Closed back headphones are designed to isolate you from the outside world. It's just you and your music. The downside is the music is a lot more 'in your head' sounding. In contrast, open back earphones have an airy and live sound to them. Unfortunately, they don't restrict the sound from leaking out. This means people around you may be able to listen to what you are listening to in your headphones. Open backed headphones also allow air flow which means they will be more comfortable when used for long periods. And on that note, make sure you try out the headphones for fit and comfort before you buy them.
If you have purchased a portable piano, you will also need a sustain pedal that will connect to the piano via a quarter inch socket. If you want to be able to gradually apply sustain to the sound you will need to buy a pedal that has the same mechanism as the pedals you find built into upright digital pianos. Other alternatives are simple on/off switches which, while cheaper, won't give you the same degree of control. Beware, some manufacturers pedals only work with their pianos so check first before you buy.
Finally, if you want to connect your digital piano to your computer, you will you need a standard USB cable that goes from your piano to your computer.