If a land lord file a claim against a tenet to a collection agency and the claim is not justified and the tenet sue in small claim court to have a judgment so this can be removed from his cred?

First you call the collection agency and tell them this is in dispute . Then get some legal advice. If you qualify for Legal Aid , go there. Even if you don't qualify, they have pamphlets for your State's Landlord/ Tenant Laws.