Time Machine

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Joined: Jan 13 2007
I have a external Hard Drive on my computer, which i call Time Machine and in the past have used. But for some reason it fails.
I open Time Machine and this is the set up.
Available: 109.18 GB of 999.86
Latest Backup: Failed
Oldest Backup; 13th of January 2011
       I press Options.
Exclude these items from backups. In there is sitting my Time Machine Hard Drive. With 890.68 GB.
Underneath i have a + and - button. I can only press the + and put material in.
Can any person tell me how i can activate or restore my Time Machine.
Thank you