The problem you're encountering has been one of the primary issues for graphic artists and designers of computers as long as they've existed. "What you see is what you get" is difficult to achieve, and requires calibration of your devices.
The easy solution is, when you're lightening your photos just make it lighter than you think it should be on the screen, so that when it prints it is lighter overall. However, ideally it'd be nice to have the photos on the screen look closer to what they will when they print.
One option built into the operating system is to do a color calibration on your monitor. Do this by:
1) Go to Apple Menu->System Preferences...
2) Click Displays
3) Click Color
4) Click Calibrate...
5) Follow the on-screen instructions for calibrating your screen
This will change how things look on the screen, which should hopefully be closer to what will print.
iPhoto is pretty limited in its photo editing capabilities. You might consider an inexpensive piece of photo editing software called GraphicConverter <http://www.lemkesoft.com/>
;. It does a great job for a reasonably low price.
If you want to stay in iPhoto, one thing you can try to lighten the image when you're editing is to go into Adjust for the photo, and find the graph with the arrows for 0% and 100%. If you drag the 100% arrow to the left, then it will lighten the image.
These are not perfect solutions by any means, but they should provide you with better images than you have now.
I hope this helps! Let me know if you have any more questions. Kind regards,