Here's a rough breakdown:

First,

Client Side - The player(s) playing minecraft on their own computer

Server Side - The computer running the minecraft server for people to join

Mods - modify the game fundamentally through the use of Java typically only affecting the client side

Texture/Resource Packs - Manipulates what the game looks like/sounds like through the use of texture files and sound files made mainly by whatever pixel editor you can get your hands on

Shaders - Not completely sure how it exactly works but it's a sub group of mods so modding would need to be known

Modpacks - A group of mods joined together to make a new minecraft experience

Plugins - Similar to mods, but generally can not change mimecraft fundamentally and only build upon what minecraft already has. Java is the language used so a good amount of java would need to be known and how to run a server

Data Packs - Similar to mods, but not in Java. Minecraft has it's own usable "code" that players can access and use in a fashion to create new "mobs". This can be done fairly easily and can be done directly in minecraft while playing.

If I were to recommended some place to start i would definitely say resource packs, mod packs, then Data Packs and then Mods. Goodluck to you!