how do I go about putting lien on someone's property

If by "someone owes me 12,000" you mean that you have a judgement against someone for $12,000, you can record that judgment as a lien against that person's real property.  The exact procedures differ in different jurisdictions, so you should consult the office of the County Clerk in the county in which the property is located.  If you do not yet have a judgment, and have no agreement with this person in which that person has given you a security interest in his property to secure his debt, you will have to obtain a judgment by suing the debtor.