This guide on the wiki has everything you need to get started, but here are some quick highlights.
- Stardew Valley's mod loader is called the Stardew Modding API, often shortened to SMAPI
- Once SMAPI is set up following the guide, the most common place to find mods is Nexus Mods
- If you're playing on the new 1.6 version of the game, you'll want to make sure your mods are also for the newest version, as 1.6 changed a lot about how mods work. You can either check if the mod in question is tagged as compatible or broken in 1.6. If neither is tagged, click over to the posts tab, where someone will probably say if the mod still works for them, or might link to an unofficial update.
- Most mods will have framework requirements. Nexus will let you know what the required mods are when downloading, make sure to pick them up as well, and that they are also compatible with the new update.
As far as picking your mods goes, I'd recommend
Salmence's video on picking your first modlist, as well as the rest of his
mod showcases. Again, here are my personal tips.
- Generic Mod Config Menu is, in my opinion, completely necessary. It will make configuring your mods so much more convenient and intuitive.
- If you just want some small tweaks or quality of life additions, some good picks are NPC Map Locations and No Fence Decay Redux. My personal favorites are To-Dew and Adopt with Krobus.
- If you want to see more of the valley's NPCs, an essential choice is Canon Friendly Dialog Expansion. If you want more drastic reimaginings of character design, I couldn't recommend Diverse Stardew Valley more.
- If you want whole swathes of new content to explore, there are 3 big expansion mods to choose from. I wouldn't recommend having more than 1 of these in your save while just starting out. Salmence has also made in depth showcases about each of these, if you're interested.
- Stardew Valley Expanded is probably the game's most popular mod. It features large changes to Stardew's existing maps, adds 15 giftable npcs, 6 of which can be romanced, and 3 optional farm maps to pick from. It has some really interesting expansions on canon content, notably in its Joja storyline. Because it modifies a lot of existing parts of the game, this is the most likely of the 3 to run into incompatibilities with other mods.
- Ridgeside Village is by far the biggest of these 3, and the one I'm currently playing with. It adds an entire other town, with over 50 npcs, 22 of which can be romanced, along with tons of new forage, recipes, fruit trees, and fish (3 new legendaries).
- East Scarpis the mod I know least about of this whole list, but from what I can tell, is the most customisable of the 3 mods. The new areas added are mostly vacant, with selected NPCs that can be downloaded separately. The NPCs have a wide variety of backgrounds and connections to the valley, so there's someone in here for everyone to befriend.
- One of the mods' optional characters, Jasper, does not require East Scarp, so I didn't actually realise that he was a part of this mod. I would highly recommend adding him into your game, he's great.
I could go on and on about particular mods from my list and why I love them, but the world of modding is so broad I could never cover everything. There's so much more I wasn't even able to touch upon, such as UI mods and recolors, which highly I'd recommend looking into for yourself. Happy modding!