Pro tip:
On your first day, make sure to get
1) Plenty of wood
2) Enough food to last you a while
3) Enough wool for a bed
4) Get some sugarcane
5) Make sure you get at least 3 or 4 saplings of your choice.
That way, you can make farms underground, allowing you to have plenty of wood, and food to survive down in these caves.
Another good tip is that since u cant step outside, look for endermen that could be holding a grass block. Kill it, and u can spread the grass to dirt to start spawning animals.
Good luck! ;)