Start here:
(the below format is property of whywefight, thanks for that.)

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no (valid) collateral, no loan.

Things you need to know:

[EDU] The Rule of "No Collateral, No Loan" - IGNORE AT YOUR OWN RISK!
How to sign a message
Bitcointalk Escrows

Read all of them and after educating yourself ask again.

You can lock this topic at the lower left of this page. thank you!