The Creation Kit is a free mod's editor for Skyrim developed by Bethesda and you can install it through Steam.
It has many useful tools for mod developers to use and with it you can change or add almost anything into the game.
Always rely on your imagination, creativity and patience, then you might have a lot of fun modding games.
Required by many mods, SKSE is a tool that adds more scripting possibilities to Skyrim.
Without it, the use of some mods is simply not possible.
Although BOSS is outdated and no longer supported by its developers, it is still a nice and easy-to-use tool to automatically fix the load order of your mods.
Note: If you want a more updated tool, use LOOT instead.
Loot is the abbreviation of Load Order Optimization Tool.
It is one of the most updated and reliable tools to check and properly organize the load order of your Skyrim mods.
TOOLS
TES5Edit is a tool that checks for conflicts between the mods you have installed and fixes them, or points you into the right direction to do so.