you're outside at night on your first day
Don't blame yourself. Maybe there wasn't a tree for miles, or you took a little break and came back to find it was getting dark. But, of course, what do you do if you have no shelter?
1.Find a good spot which is open to the sky, so zombies on top of your house will burn in the daytime.
2. Yes. I know. It's the Golden Rule of Minecraft being broken.
Dig straight down,with your fist. Only 3 blocks though. That'll be enough.
3.Dig out the area a little.