Slimes are one of the rarest mobs in the game and only spawn on Easy difficulty or above. They can spawn in well lit areas, so make sure to do this as the amount of other mobs spawning will decrease the chance of slimes spawning. This is only in slime chunks so there's no need to place a torch everywhere on the surface.
Remember to look-up how to find slimes on YouTube before clearing away a massive area and save hours of unnecessary work.
Slime Finding Tools
There are a few different applications to help locate Slime Chunk locations in different world seeds. There are several YouTube tutorials, but the programs are fairly self explanatory. The most popular Slime Finder is at extension.ws/minecraft/slimes.html. There is also an Android app named "Slime Finder for Minecraft", that was developed by Saranomy. This is a free app that is available on most application sites.
Slimes are abundant in Flatworld as its only 4 blocks away from bedrock and very well lit up.
Slime Farming Tutorial