Where do i have to put the Mods ??

[#NEOИ]

...........

[Vroomi]

It depends on the mod.

[The Y'All Of Us]

Each mod should come with installation instructions on its' download page, you should check there.

Otherwise, specifying which mod you're trying to install would likely yield more helpful answers, as mods come in many forms and could be applied just about anywhere in your game files depending on content altered/added.

[Fizzgig]

1.
Some mods are standalone that will usually replace one or more of the games xnb files. These are usually in the (Steam Path)... \steamapps\common\Stardew Valley\Content\Data

For example one mod replaces the Fish.xnb file so you find it in data, make a backup of the original, copy your new replacement and rename it if needed. Look for correct instructions on the mods discussion page.

2.
Other mods can utilize the SMAPI library. Mods like this require you to first get the SMAPI and drop it in the main Stardew Valey folder (Steam Path)... \steamapps\common\Stardew Valley

SMAPI has with it an exe file and a dll file along with a new folder called Mods. So if you drop all 3 things in the main folder you will now have a mods folder to add new mods to. So again you need to look at your mods instructions to figure out what you are supposed to do.

Those are generally the 2 types of mods available, but instuctions will vary for each mod. So read everything carefully.