Hi there, thought I’d write a little guide on mods, since it’s really improved my Minecraft experience since I started dabbling in them 
The main purpose is to give you a rough idea of what’s required to make mods work, and to give you a look into what’s possible, with some game changing examples. Please keep this list of allowed mods in mind when installing mods, and ask when unclear.
The following is based on my experience, and I might be wrong about some things, let me know in the comments, let’s hone this thing 
Platforms
First off, mods on Minecraft don’t always work by themselves, they often need a framework to work on. There are two big ones (and probably a bunch others): Optifine, and Fabric. I’ll be focusing on Fabric, since the mods which work the best for me run on it.
Shaders and graphics
Optifine is known best for 4 game improvements: increasing FPS and the graphics overall; the zoom; shaders; and dynamic lighting.
On the Fabric platform, the graphics improvements is Sodium.
Logical Zoom has been created by LogicalGeekBoy as he was toying with mod creation.
Shaders are quite well-done, running better than Optifine for many, with Iris.
Of course, this mod allows Shaders to be used, but doesn’t ship with any. To actually start using shaders, there’s a lot of them out there. I prefer Sildurs, but the world is your oyster! When using Iris, it’s ridiculously easy to toggle shaders as well.
Lastly, the dynamic lighting can be achieved through a few different ones, but I personally prefer Lambda Dynamic Lights.
HUDs, overlays and minimaps
- Inventory HUD +: a highly customizable overlay, can show your inventory, equipment, buffs/debuffs, in custom spaces.
- Better Mount HUD: shows your health and hunger while on a mount
- What The Hell Is That?: can display a pop-up with information on what the heck you’re looking at
Item/entity interaction alteration
- Mouse Wheelie: improve transferring items between containers, make items scrollable between containers (depending on which way you scroll)
- Easier Villager Trading: streamlines interactions with villagers
- Grass Bypass: You didn’t want to hit the grass, you wanted to hit the enemy!
Installation tips
If you play on multiple servers (maybe even if you don’t), you may want a Minecraft folder per server. This is easily managed by using tools such as MultiMC.
Once installed, you create an instance, r-click it, and Edit Instance. Under the “Version” tab on the left, you can click “Install Forge”.
FYI
To clarify: “Install Fabric” will install only the necessary files for Fabric to load, it won’t install the Fabric API, which some mods also rely on. This API can be added by installing Fabric from their website, as linked above.
There, you can click “Loader mods”, and on the right once more, click “Add”.
Extra tips
If your game crashes, scroll through the Minecraft Log. (A random guide I found on how to find these logs here). It’s a lot of text, but just scroll until you see something as recognizable text, there are usually error messages that say quite clearly what’s wrong, such as a missing library. Some mods require other mods which function as libraries, which can usually be found in the information tab of the mod pages.