How to keep lens from fogging up and how to clean it (nikon DSLR)?

My room is really cold (military base so I have no control over room temp and no idea how cold it is in here) but the weather outside is usually really humid and warm. I stepped outside today and soon noticed I couldn't take pictures b/c of foggy lens.
The condensation cleared up on it's own soon after I came back inside but i'm worried that this will eventually damage my camera/lens and would like to know how to prevent this problem in the future.
This is my first owned digital camera and I plan on buying a camera bag eventually. Will the camera bag help?

Added (1). I forgot to mention the fog was inside the lens where I would need to take the lens apart to clean it myself. Not sure if that info is something you will need to know.

It's a common 'phenomenon' in photography… When taking equipment from a cooler 'climate' to a warmer 'climate'… Just get into the habit of allowing a few minutes for your equipment to 'climatise' to the new temperature, it's good practice to carry a lens cloth just to give the lens a quick wipe before starting a shooting session…

… If you were to leave your equipment 'idle or unused' for any length of time, then 'fungi' might begin to grow… If you use it a lot though and keep it clean you should be OK.

Camera equipment is quite 'tough' and although it isn't the ideal scenario for your equipment… I'm sure it will survive without any problems.

@ Additional Details: sometimes the viewfinder eyepiece and mirror will be affected too, just allow a couple of minutes for your 'gear' to 'warm up'… It sorts itself out and is really difficult to prevent.

It's a normal condition. I run into this all the time when I'm vacationing in the Caribbean when going from inside air conditioning to outside.

You can do two things.

First, just leave the lens caps on and after going outside, let the camera temperature stabilize for 15 minutes prior to using it.

Or, find a heating pad and set it on low, and put your camera bag on it for 15 minutes before going outside.

Keep your camera in a bag and when you want to bring it outside, bring the bag out with the camera inside. Let it stay like that outside for about 2 minutes before you bring out the camera.

Taking the lens apart to clean it must be done in a dustfree environment. You may be doing more harm than good if you do this habitually.