Optional: You can zoom in to increase your accuracy. To zoom in, press Ctrl+(+)/Cmd+(+) and to zoom out, press Ctrl+(-)/Cmd+(-).
Now drag your mouse cursor over her dress. Based on the edge detection, Quick Selection Tool will automatically set its boundary. In case you want to read about Quick Selection Tool in detail, you should read my tutorial on Quick Selection Tool.
After few seconds, or minutes, the selection should be completed.
Now we need to invert to selection so that everything would be selected except her yellow dress. To do this, press Ctrl+Shift+I/Cmd+Shift+I. Now you should be seeing those marching ants in the four edges of the image.