Landlord/Tenant disputes

Questions:

• In my tenancy agreement it says my landlord has to carry out repairs and he hasn't. What can I do?
• My landlord has changed the locks and I can't get in, can I break in?
• My landlord has evicted me and I dont think it was legal, what is the proper procedure? / I want to evict my tenant, how do I do it?
• My tenant has not paid his rent, what can I do?