How can I turn the MIL light off in my 1999 Nissan Quest without visiting mechanic?

Autozone and have them read the code and then clear it for you. It will most likely come back on if there is an underlying problem that doesn't get repaired.

You can also buy your own scanner, that reads codes and clears them, if you work on your own cars. I got mine at Sears for about $110 or so.

Re: 1999 Nissan Quest
Can I replace head light bulb without opening up the whole box?
I can see condensation on the inside side of the glass. Can this contribute to the fact this bulb goes more often out than the other?:confused:

Pop the hood. Find the wire harness that goes to the headlight bulb in question. It's accessed through the rear of the headlight assembly in other words.

Sounds like you need to install a used assembly if your getting water inside. That bulb burns hot and having water get onto it will burn it out.