Start a game on Survival and make the difficulty Easy.
On normal more monsters will spawn but will be more difficult to kill. On hard even more monsters will spawn and they will be even harder. You can make the difficulty whatever you want really...
During the day just go around getting wood, iron, gold, diamond, etc.....and make a sword
When it is night time just be outside and find a monster and hit it until it dies.
Some of the monsters are
Zombie (common)
Skeleton (common)
Spider (common)
Creeper (these blow up so hit them once and run) (common)
Slime (common)
Ghast
Blaze
Magma Cube
Cave Spider
SilverFish
EnderDragon
Wither Skeleton
Zombie Villager