My Macbook Air's small hard drive is all filled and I can't open mail. I need to copy some of the mail files to a flash drive and change my mail setting so it won't copy emails to the disk to reduce space..and allow computer to work.

I have a Mac Book Air.. mid 2012.. 120 Gig HD. All filled so programs won't open.. only Safari will. I can't get into my mail. 70 G of drive is Other.

39 G are Applications. So I have to get the Other smaller and copied to a flash drive. I have copied all files in the Documents folder that I recognize to a flash drive.
I think my problem is that I have been saving my emails (mostly gmails) in folders on the computer and I need to somehow open Mail so I can change whatever setting I set up to do this so I can keep memory free for the computer to work. I want to store all the files I can on an external flash drive or HD if possible because this computer has such a small HD. Currently Mail will only quit unexpectedly and therefore I can't get in to delete or change things.

I have been working with an Apple person but so far no success.

I would very much appreciate whatever help you can provide. We have tried all the normal steps... emptying trash... emptying caches..

HELP!!!!!

Thank you in advance for any assistance you can offer..

Fran