@laura_owen24 I have no idea where you are at with your understanding of pd, so I will start with the very basics. I apologize in advance if this is old hat for you. It also sounds like you want to figure this out on your own, so I'll will try not to give too many spoilers, unless of course you need more help.
Look into the [key] or [hid] object. This will grab keystrokes from your keyboard.
Then you need to setup some sort of selection system so that when a certain key is pressed "something" happens. Look into the [sel] object (short for 'selection') for this. BTW you can right click on any object to bring up its help file which will explain what it does an provide some examples.
Then you will need to decide what the "something" is that you want to have happen. Since it sounds like you want to convert key strokes on your keyboard into notes that correspond to a Western musical scale, you will probably want to convert your key strokes into MIDI note numbers or frequency values for an oscillator. Look into this page: http://newt.phys.unsw.edu.au/jw/graphics/notes.GIF , the [mtof] object (MIDI to frequency conversion), and the [osc~] object.
As for transposition, try to work out what sort of mathematical operations you will need to do in order to change the frequency values, or alter the data that you get from [key].
Let me know if I can explain anything better or if you need clarification on any of the points.