Call Your Landlord
Assume the best. If there's a problem, a friendly call should set the problem right. At this stage you want to stay on good terms with the landlord and get the problem resolved. But once reported, the landlord needs to deal with the issue in a timely fashion.
Write a Letter
If there are problems that you are going to wish to repair and take off your rent - or habitability problems that will cause you to withhold rent - you absolutely need to state the problem in writing. And keep a signed copy.
Repair and Deduct
If you've made a demand in writing for a substantial problem (if it's something inexpensive, why not just take care of it without making a stink?), you have a right to repair and deduct the cost from your rent. If your landlord attempts to evict you for nonpayment, repair and deduct is a solid defense. If a landlord evicts based on a valid repair-and-deduct, that is an illegal retaliatory eviction.
Withhold Rent without Repairing
You can only use repair-and-deduct once a year, but you can withhold the rent if the problem is too severe to have it fixed yourself. That is, the problem makes the unit uninhabitable. If you are evicted, you have a total defense for nonpayment of rent - if you have given written notice and a reasonable time to repair.
Call the Authorities
If the unit is in really bad shape - no heat, lots of mold, dangerous rot - you should call the authorities. If it's bad enough, they'll red-tag the place and tell you you need to get out. If that happens, the landlord needs to pay your relocation fees.
See a Lawyer
If all of the above has happened you likely have a pretty powerful case, and you should talk to an attorney about asserting your rights.