• Command (Mac) / Control (Win) + Shift + E to display the Export dialog box.
• Command + Option (Mac) / Control + Alt (Win) + Shift + E to Export with Previous (settings).
• And, don’t forget, if you save Export presets, you can Control (Mac) / Right Mouse (Win) -click on an image and select the saved preset from the context sensitive menu!

i wanna become ACE (Adobe Certified Expert ). i just need some advice.
which topics i should focus on.
There are some study guides here:
http://www.adobe.com/support/certification/exams/