Ok first off I'm going to assume you are playing the Skyrim: Special Edition ("SSE", the one you can now buy on steam that includes all DLC) and not Skyrim: Legendary Edition (also known as "Oldrim", which used to be the basegame skyrim + dlc sold seperately), as these are treated as different games in the modding scenes (mods for SSE rarely work for Oldrim and vice versa).
For starters, you'll want to make an account on the
Nexus and download the mod organizer of your choice (Vortex, MO, etc etc) as these make installing and updating mods alot easier.
Regarding mods themselves, you'll definitely want the
Unofficial Skyrim Special Edition Patch to fix most of the glaring bugs, aswell as the latest version of
Skyrim Script Extender (SKSE)(Make sure you download the version for Special Edition). That's really the only essential mods you need, as most other mods require one or both and the Unofficial Patch fixes most of the glaring bugs.
also keep in mind the three rules of modding skyrim:
1. Don't uninstall a mod mid playthrough. Make backup saves before you install a new mod and roll back to those if you uninstall it.
2.
Don't uninstall a mod mid playthrough.
3: Turn off autosaves and quicksaves and get into the habit of using manual saves, as autosaves/quicksaves can break scripts and cause your savegame to become irreversibly damaged and CTD later down the line.
Now I will share some of my favorite mods:
SkyUI completely reworks the atrocious Skyrim UI and adds functionality for ingame mod configuration menu's (MCM's) that many mods include. No downside to installing this.
Alternate Start allows you to skip the Helgen Opening Scene and customize your Start more than vanilla Skyrim.
Diverse Dragons Collection adds more dragon variety.
Apocalypse - Magic of Skyrim adds hundreds of new spells and effects and fleshes out the meager magic system of vanilla skyrim. (WARNING: Script intensive, make sure to turn off autosaves)
Ordinator - Perks of Skyrim reworks every perk tree in the game and adds alot of new options to build your character. Also compatible with Apocalypse with a compatibility patch. (WARNING: Script intensive, make sure to turn off autosaves)
I deliberately left out any graphics mods or body replacers (
*cough*)
as those are really down to personal preference.
One last thing, when using many mods, you can often lose sight of compatibility issues and load order (something that most Mod Organizers do automatically but sometimes you need to change something) so I highly recommend you use
LOOT to make sure you are not using incompatible mods.