The first thing that you need to choose is the Ad type (Ad unit or link unit). Based on the Ad type you choose, you will need to specify the type of ad (text, image, etc for Ad unit) or number of links per link unit too. Google AdSense program allows you to use a maximum of three ad units and 1 link unit per web page (and you must use both, in order to maximize your AdSense revenue).
The next thing is to choose the Ad layout. There are various layouts available but large rectangle (336×280) and wide skyscraper (160×600) are preferred over others (since they help get more impressions for you and hence help you make more money).
Customizing the colors of various parts of your AdSense ad is the next thing you need to do. Even though customizing colors is optional (as far as the Google AdSense program is concerned), its strongly recommended for making your Ads attractive and more effective. Your choice of colors must be governed by the look and feel of your website so that the Ad doesnt look out of place.
These are the essential customizations that you surely need to do before you can start using Google AdSense Ads on your website.