One of the best and easiest to use tools for digital photos is the cropping tools, which cuts a desired section out of an image, trimming off unwanted parts (at the edges). Using Picasa, cropping is easy. In the editing view, click Crop. There are three default sizes(in cm 10 x 15, 13 x 18, and 20 x 25). The fourth option, Manual, is the one I use most, it allow you to click a point in the shot and the drag the mouse pointer to selection the portion that you want to crop. The rest of the photo will go grey. Click Preview to see the crop. If you like it, click Apply; otherwise click Reset and try again.
Why a cropping tool is so good? It allows you to improve the composition of your photo. Most digital cameras now do a wonderful job of exposure and focus, but none as yet include a magic button to make your image interesting. That is in my opinion now the biggest difference between a mediocre photographer and a great one. There is plenty of advice on the web on the basics of good composition. Sometimes you do need to use your judgement and not follow the rules blindly, for example I left the horizon in the middle of my final crop, because there were enough shapes and lines to draw the viewers eye into the image.
Most Photo software, including the software bundled with your camera, and even photo kiosks will have a cropping tool. Many cameras now have built in cropping tool. So go now and try cropping that photo you liked but where disappointed with when it was printed.