If you're looking to make a section of an image transparent first go over underneath the draw section of your tool panel (on expert mode) and click on the eraser tool. From here you can select a regular eraser, a background eraser, and a so-called magic eraser. The magic eraser works within the natural lines of your photo and erases whatever is contained in the area you use it on. It is also important to remember that the magic eraser's tolerance works a little differently. The lower the tolerance, the closer in color (to the area which you select) the pixels are, which the eraser then removes. Another tip that I have picked up is turning on caps lock while utilizing the magic eraser. This transforms your mouse into a single point no matter what tool you are using, allowing you to see more precisely which color pixel you are selecting and erasing.
The magic eraser can be helpful if you are trying to make a large block of your photo transparent, however without lots of practice it can be hard to judge how to correctly adjust the tolerance to achieve the right eraser margins. Thus I would recommend manually using the background eraser for smaller and more complex sections. If you are editing on a laptop I also suggest using a mouse when operating tools like this because it gives you more control. But you can also always become a track-pad master like myself.
Once you have set the size and tolerance (opacity) of your eraser you can erase away to your heart's content. After completion you may notice that saving your image normally, as a JPG, undoes all of your hard work and saves any transparent area as white empty space. This can be frustrating but is easily fixed. Simply change your format upon saving to PNG, if using the save as feature, and to PNG-25 if saving for the web, since the PNG format supports transparency. Happy Editing!