Living underground is a fairly simple thing to do and a very safe thing to do in minecraft. In order to properly live under the surface of the world, you need to have:

Many pickaxes(mining the large space you'll need)
Many torches(1 per every 5x5 square)
A monster spawner(unless you don't like to hack, otherwise, make mushroom stew)
Time

1. Mine out an area that is sufficient enough for you, my friend and i mined out 1,850 blocks in order to live underground, but we had a few hours to spare.
2. make sure it is lit or you play on peaceful
3. Keep food supply high, hunger low(don't be starving constantly)
4. Have a mine system somewhere(we made this because of mining)
5. Preferably have a friend assist you
DOWNLOAD LINK TO WORLD:
www.mediafire.com/?2w2iz2gfear121i

P.S. the texture pack i used is LB Photo Realism 64x