Note: Depending on which Minecraft instance you have installed, depends on the choices of commands you have. The following are commands per Minecraft instance:
Minecraft/Vanilla commands can be found here.
Craftbukkit commands can be found here.
Spigot/Paper commands can be found here.
SpongeForge commands can be found here.
Note: You will need to OP yourself or have the right permission to use a majority of these commands.