pick a loader first, fabric or forge/neoforge. every mod you download has to match both that loader and your exact game version, that's what trips most people up.
1. run the loader installer for your minecraft version
2. open the launcher once with the new profile so it makes the folders
3. drop the mod .jar files into the mods folder (on windows type %appdata%\.minecraft into the run box to find it)
4. launch with the fabric/forge profile, not the normal release one
if you go fabric, most mods also need Fabric API sitting in the same folder. and only grab mods from modrinth or curseforge, the random reupload sites are where the malware is.