The private key is what enables you to spend the BTC associated with the address.

http://bitcoin.stackexchange.com/questions/7536/how-do-i-export-my-private-keys-from-my-bitcoin-qt-client

http://www.blockrun.com/faq/how_to_sign

And a sample signed message that I posted yesterday:


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Test message.  Ignore the dashes above and below when you copy and paste this to verify.

If you can verify this, you've probably figured out enough to sign messages with your own keys.
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Signed with:  1grnbrg3Ea4t6bxHvQKRvorbBeLNDXv2N

Signature:  HKT07NvQ31Mz3mpMUD+38RSnEPa0/HyrE0zx8qse5THhq3SDrsBHVLt+UogA1bsxqwy+MhYkRy1Uc+ph+627bXI=



grnbrg.