Zombies are the most common Minecraft enemy you'll meet while crafting Minecraft. They make a loud noise so you'll be able to hear them coming from a mile away. They don't hurt you too much but you should kill them as soon as you can just like any monster. They also have a chance to drop any random item that exists in the game but the rarer items barely ever drop.
Creepers are the iconic enemy in Minecraft. They're the green things that explode. The explosions are a lot less dangerous these days. They used to destroy the environment and would break your house but that's not the case anymore. They can still hurt a lot though and explode when you attack them, so you have to kill them quickly. They also drop gunpowder which is a great crafting ingredient.
Spiders are an enemy that are around during the day (most enemies die when hit by sunlight) but you don't need to worry if you see them during the day. They're only aggressive at night. You need to be careful around them when they're aggressive as they can jump and attack you, and can run up walls made from wood. Either build a roof or make a home out of stone. They drop some crafting materials and are worth killing if you run into them.
Along with Creepers, Skeletons are one of the more problematic enemies. They have bows and arrows that allow them to shoot at you and attack you from a distance. They also rarely ever miss if you don't keep moving. There's a chance you might be killed before you can reach them and some can be hard to spot at times. Of course you can always get a bow of your own and shoot back at them. The bones they drop are pretty useful too and you should remember to pick them up. They can be turned into bonemeal that grows plants and can be used to tame wolves.
Wiches are perhaps the most annoying enemy you'll encounter but they're also very rare. They throw potions at you which poison you and can do a lot of damage. They also have a lot of health and take a while to kill. You shouldn't try and take one on until you have some armour and a good quality weapon.
While there are a number of different enemies they mostly only come out at night and won't appear if you light up an area well. You shouldn't be worried about them appearing in your house or anything. You can also change the difficulty setting to peaceful, or play on creative mode, to make it so there are no enemies at all.