Select Sharpen Edges from the Effects menu, and choose a value. The image will appear sharper than before. The best way to learn other available features is to experiment with them on sample images. Some of these features will alter the current image, while others will create a new window and produce the intended effects on the image in this new window, preserving the old image as it was originally. Experiment with these tools to get a feel for how they can be used.
Most tools are self-explanatory.
Some, however, are not. The Eye Dropper tool lets you choose a color in an image to use as the foreground color. The Transparent tool lets you select a color in an image to be completely transparent.