Start with the pieces - pick something small, make it work then add to it.
For example, I made an RF remote control. First I got 2 arduinos talking to each other using standard playground code, then I changed it to send the characters I wanted, then I added reading in the button presses from a keypad using the keypad tutorial and changing it from 12 button to 16, then I added going to sleep and waking from an interrupt created by any keypress using ideas from other posters, then changed to a lower voltage arduino and had it run from a Li battery, next I am adding a battery charger circuit (waiting to figure out some resister needs for another project and will order panel mount power jacks for the remote at the same time - seems like UPS manages to make a buck no matter what!). Somewhere in there I made it fit into a clear plastic case that turned out to be pretty crappy, the hinges gave out, and I rebuilt it into a sturdier box.
So start small, build up. Submit your code and schematics when you ask for help.