In minecraft there are many types of hostiles that can be found. Different hostile has different types of attacks and abilities. I will give you a link to a website that explains about it.
http://minecraft.wikia.com/wiki/Category:Hostile_Mobs

Here's the picture of one of the hostiles called Ghasts taken from http://www.pcgamer.com/previews/minecraft-halloween-update-preview-meet-the-ghasts/

   A first night might be difficult for new players but you should manage somehow. In your first night, you should be careful as you might have no decent equipment to fight a swarm of hostiles. Hostiles only spawn at night (at night they spawn every where) or at caves (even at day). Hostiles will try to attack you on sight, there are many types of hostiles with different types of attacks.

   Hostiles that spawn at night in the field will stay until daytime and hostiles reacts differently to sunlight. I will talk about this in another post. But you should be fine if you are careful. In your second day, you should find materials to build more decent tools to survive better in the open. You should find stone first to build a stone pickaxe so you can farm for coals to make torches for better vision at night and deep in caves.

   Deep inside caves hostiles can spawn because of the light level inside the cave. But, ores that are required to build even more decent tools and weapons are mostly found in caves and some are very deep. Just like real life, some ores are harder to find than others. Ores can be melted to ingots by using furnaces.To make those here's a link to a website guiding you in crafting (making tools and weapons):
   http://www.minecraft-crafting.net/