When selecting the size of your Google Adsense code the 300x250 is by far the best performer
. This square box works best when placed in the middle of the content towards the top. I commonly place the 300x250 ad block under my title and on top of my first paragraph. Sometimes I put the 300x250 ad block under the first paragraph if it is short but never below the second paragraph. Banners and skyscrapers may look better on your page but will perform much worse than the 300x250. Remember why you're publishing your content; you want to make money not look pretty.
When selecting colors for your Google Adsense ad block you want to blend the ad into your page. The best practice is to make the ads look like content so users will be interested in clicking the ad. I've had great success using a basic white background and black color text on my pages. I then use the default blue for my Adsense text and use "White" for the border of the ad block. This causes your square to look like it has no border thus blending it into your content. If done right your Google Adsense block should look like you put links on your page your visitors might be interested in.
Google allows you to assign a "Channel" to your ad block so you can track the success. Be sure to assign each new site or subject a unique channel so you can tell what subjects are popular and pay well. Using channels is also a good way to test various ad colors and sizes but if you use the 300x250 you can generally be assured that you're using the most successful size.
When starting out with Google Adsense it will be very tempting to click your own ads, some do this because they are interested in the ad but some are cheating. In both cases you will likely get kicked out of the Google Adsense program if you click your own ads.