trouble with gmail, need help.

help me! what does gmail's all mail means? i only want to keep my mails under sent mail folder, other mails except folder sent mail should trashed, how to operate in the aim to clean my gmail which now consumed 1g and more? help me! is there a keyboard shortcut for delete and go next mail? i previously took for granted as "]", but today i know its only remove label and go next mail. help me kind people!