I've recently started playing Minecraft, and I only have a little experience with enemies (I've been playing with the peaceful setting so I can figure out what the heck I'm doing). When I did play some survival mode, I happened to be at the top of a pretty large cliff when night hit, and this seemed to protect me - the baddies just piled up at the bottom of the cliff.
However, it seems like this probably only worked due to pathing flukes. The terrain behind me was pretty easy going, so I suspect if enemies spawned in the right areas, they could still come up and visit.
That said, how can I set up a quick protective shelter for the first night or two? The days don't seem to last very long, so I don't expect I would be able to generate a lot of high-end materials, tools, etc. to work with.
Also, the zombies caught fire nicely when the sun came up, but there were still some spiders and some other things (creepers? - they were explosive) that didn't go away at daybreak. Do I need multiple exits for my shelter or some other way to avoid running into all the enemies outside the front door that have accumulated overnight?