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/

First Play

     In Minecraft, there's no definite end game, but there's some achievement that can be completed to make the game more interesting and have a story line. This achievements does not have to be completed, but if you want to you can.
   
     In your first day, you should find some wood and a place for shelter so you can survive your first night. At night, hostile NPCs will spawn more frequently, especially in dark places. These hostile NPCs will attack if they see you or you attack them. If you doesn't have a decent weapon it's better to find a place to hide and wait for the night to pass. When morning comes, some hostile NPCs like Spiders and Creepers will survive, but NPCs like Zombies and Skeletons will burn and dies.

     But, if you are not confidence, you can set the difficulty to peaceful so that no hostile NPCs will spawn, either day or night. This way, you can survive better and prepare better for the next few days when you want to set the difficulty to normal again. There's nothing wrong in playing peaceful difficulty all the way through, but for some challenge I recommend normal difficulty.

    Either you play normal difficulty or peaceful difficulty at first, I suggest that you prepare yourself before setting out to find materials at night.

Tutorial

  For you who is new to the game, I found a video from youtube that might help you to do things to survive the first couple of days in Minecraft. I didn't make this, but I think it helps to get to know the basics.
  Here's the video link:

I hope the video can help you to get through your first day in Minecraft, happy playing, good luck and have fun :)

Download

       This post is going to give a download link for MinecraftSP, a free minecraft game in case some of you didn't have a minecraft game yet. It is not quite like the official as it is a launcher, but it plays minecraft just fine and you can enjoy most feature normally.

        Here's the download link:
         http://www.4shared.com/zip/ROBWwlX7/minecraftspexe.html

An ingame screenshot:

     

                








source:
http://static.filehorse.com/screenshots-web/online-games/minecraft-screenshot-02.jpg