Basics

To edit photos you'll use the tools in the Bitmap section. Here you can see where some tools are grouped together...

• Marquee
• Lasso (also Polygonal Lasso)
• Magic Wand
• Eraser
• Blur/Sharpen/Dodge/Burn/Smudge...
• Rubberstamp / Replace Color / Red Eye Removal

When photo editing, you need to use the Layers panel. At the top of the Layers panel, you can change how the top layer blends with the layers underneath (the blend mode and opacity %, as well as more options...

Go to the Properties panel and use the "+" button. Make sure the photo/layer is selected with the black arrow tool! Don't go to the FILTERS menu at the top - because then you can't go back later and change the settings or undo. 

Once you've applied an effect, you'll see it in a list. To hide the effect, uncheck it. To delete an effect, use the minus sign. To edit a filter, double-click on it in the list and change the settings.