Outlook 365 mailbox full error

I cannot get rid of the following message 'your mailbox has reached maximum size......'

I am NOT very computer literate so need basic instructions please.

• I am running outlook as part of my personal Office 365 on a windows 7 Pro system.
• It is not a shared account, used only by me from home.
• I have checked my outlook email size which is 11,139kb. My gmail size is 3,442kb.
• I have cleaned out my deleted AND sent emails.
• New emails keep arriving but I cannot send any?

Thanks,

Kev