set small budgets and test
Set small budgets and see what happens. Do multiple versions of the same ads. Look at what keywords are best and use those exact words in your ads. For example, if you search for 10 megapixel digital cameras and see an ad for "electronics" it won't be as good as an ad that comes up saying exactly
"10 megapixel digital cameras, 20% off!"
Use different ad groups that match up with what was searched more specifically. Watch all of the google resources including easy to understand videos to make sure you are doing everything right. You might find that a slight change in words makes a big difference. Just watch closely so you can stop getting charged before its too late.
I've been experimenting myself a lot this week for google ads
I hope that helps. -Jeff