Do a search for $25 Googlke AdWords credit and you might be able to find a coupon.
Before you begin, understand that Google AdWords can be a tough nut to crack. If you are buying traffic for an established company/website with a big budget, then it can be pretty easy.
However, if you are advertising a new site, then things will be a little more difficult. When you go to Google AdWords, make sure you do a search for "quality score" and understand what that is.