I place the mod files in my Skyrim data folder, start up the game, but I can't see any evidence of the mod working, even when I check the data files on the skyrim launcher there are no files to be checked. I've made many various attempts at downloading mods in Skyrim and often it has caused me to have to reinstall the entire game because something wrong happened. I've also tried modding Oblivion in the past but the mods never showed up or didn't work no matter what I did. I've tried many mods and the last one I tried was some Immersive Water mod for skyrim but the water didn't look any different after I installed it.

Can someone please help? If a tutorial video for downloading mods for Skyrim exists, then it would also be a great help to post the link.

EDIT: In the data folder, the skyrim mod files are .dds files if that's any help.

Try adding 'em (plugins) manually, including file extension, to %LocalAppData%\Skyrim\Plugins.txt. Or, have you tried http://www.skyrimnexus.com/content/modmanager/?

.DDS files (textures) should load automatically provided they're placed properly in Skyrim\Data\Textures.

DDS files are textures, not plugins, so they shouldn't be in the data folder. They should be at least under Data\Textures, and then one or more other directories depending on which category the textures apply to.

Those are texture files, and should not go directly in the data folder. Did the mod come packaged with a folder structure already intact, or are there installation instructions on the download page or in the readme file? Different types of mods are installed in different ways.

Cipscis

Ok, I put the dds files in Textures but they dont appear on the Data Files tab on the startup menu. Will that mean they won't work if I cant check them off?

They don't. Only ESPs and ESMs do. Textures and Meshes are always active unless overridden by another one.

Like I said, putting them in the Textures folder doesn't work either. They must be in a specific folder, depending on what kind of textures they are. DDS files never appear in the Data files either, you just need to place them in the correct directory and they will work.

But none of the mod instructions say what category. Also, I'm only downloading texture mods. Most of them have the same instructions: "Copy the data file(s) into your skyrim directory" or somethin.

Then there is probably a folder structure inside the package you downloaded. Make sure it matches.

In that case, merge the two folders. If it doesn't have a Data/Textures folder or a Textures folder by itself, just drop the stuff into the Textures folder. If it has Data/etc..., drop it in to your main Skyrim folder and merge the Data folders. If Textures, drop into Data and merge the Textures folders.

If the folder has the correct data structure, you should be able to right-click and drag the rar/zip/7z to the data folder and pick "Extract Here' from the right-click menu

Also, alternatively, if the zip/7z/rar has the correct data structure, you should also be able to have NMM install it for you.