Definitely graphics mods, and then you can play around with adding new armor, weapons, and expanding the skill tree. If you need a recommendation for a mod manager, Mod Organizer 2 is a great one.
For mod manager, use Mod Organizer 2. Install SKSE as a starter as most mods will rely on it. Off the top of my head, SkyUI and the Unofficial Patch are basically considered vanilla at this point, those should definitely be at the top of your list of mods to install.