The first decision is what size should my piano be ? Concert grand? Living room grand? Tall studio upright or a beginner upright? What is your playing ability?
Style is important when choosing a piano. This will become the focal point of your room. Do I want a high gloss black? Walnut? curved toe legs? Shiny or satin finish. All pianos come in a variety of colours.
The touch allows us to be expressive. How heavy does it feel? is it responsive? How does it play? These are very important questions a pianist at any level should consider.
The sound is the key. How does it make our ears feel? Is it too bright ? too soft? Too harsh? Be sure to play any note even if you don't know how to play. Just listen.