Image via WikipediaHave you thought of this question and always wanted to ask: "How do you know where to start a song when someone requests that you play it in a certain key?" The answer to this question depends on a number of things.
First, you have to know the degree of the scale on which the song starts; it can then be transposed to any key. Let's say you've been playing a tune in the key of C, and the song starts on G (the 5th degree of the scale). When you play the song in another key it will start on the 5th note of that scale. (To be able to work in this way requires a knowledge of all the scales - 12 key's worth!)
Developing your ear is another very effective way to deal with this situation. To do this, practice hearing intervals; play a C on the piano, and try to hear the note of G (a fifth away). Check yourself on the piano to see how close you are. This should be done with all intervals (3rds, 4ths, 5ths, 6ths, etc.)
Being familiar with the piece and knowing the beginning note and chord is necessary. If it starts on the I chord, you will begin on the I chord in any key; if it is the II chord the same rule applies.
To sum up, go back to the basics. Learn all the scales in 12 keys, and enhancing your ear by practicing intervals. Then, knowing the first note and chord, and its degree of the scale, you can transpose any tune to a different key.
* Great Music Resource:
Learn to Play the Piano By Ear
All the best,