I know this thread is old, but modding games can be fun regardless of having played most of it already, so might as well come with some stuff. Don't know if you are/were completely new to modding, so yeah.
You should find one for your own taste really, there's different styles of textures/etc. The
nexus is a terrific place. Make an account and you can freely use the
NMM that will handle the mods for you, most mods on the nexus has the option to download via the NMM if logged in. If there's no option, it's still easy to download manually and use NMM.
Mod Organizer is another choice I prefer, but NMM is a better choice if completely fresh to such ways of modding.
Enviromental mods, not necessarily textures, but recommended
ENB presets
Visuals and graphics
You might want to check out
loverslab.com as well, the site is mostly known for it's NSFW mods, and if that's what you like, hey, even better for you. There's also a good amount of well made mods on loverslab.com not consisting of NSFW material, regardless of its reputation.
There's a lot more to know when modding, your first save game will be far from the one you end up using for a playthrough if you're going to heavily mod your game. I recommend doing it though, it's enjoyable to figure out stuff and to get it working.