Having clean headlights and taillights helps makes it easier for you to see other vehicles and makes your car more visible to others. It's a good idea to clean your headlights and taillights whenever you get gas using the squeegee provided by most stations. You may also want to carry a spray bottle filled with windshield washer fluid and some rags or paper towels with you so that you can easily clean your headlights and windows even if there aren't any garages around.
If your headlights have condensation in them you can try driving with your headlights on during the day to try and evaporate the water. If this does not work you may need to remove the headlight housings from the car to inspect them for cracks and to allow them to dry out. You can speed up the drying process and clean the inside of the headlights by adding some rubbing alcohol to the housings, swirling it around, and then draining it out. Any cracks in the headlight can allow water to enter, you can try repairing them with glue or sealant. Some headlights have vents (small holes or tubes that allow water to escape) and condensation can build up when these get blocked, so ensure that any vents are clean.
If your headlights are yellowed or hazy you can try using a headlight restorer to clean the headlight lens and remove the discoloration. Any kit should come with detailed instructions, however the process normally involves washing the headlight well with soap and water or a special solution provided with the kit and then using a polishing compound and applicator to remove fine scratches from the plastic. Some kits include abrasive pads or fine grit sandpaper for tougher jobs. Sometimes a sealant is included to help protect the surface in the future. If your headlights are only mildly yellowed you can try using a cloth with some regular toothpaste or baking soda. These are slightly abrasive and should work much like the polishing compound, however they may take more time and elbow grease.
No matter which kit you use the discoloration will most likely return within a year or two and you will need to repeat the process, so learning how to clean headlight lenses now can really pay off.