Depends on the mod. Texture mods just override textures, but with -for instance- weather mods it's one or the other. Just carefully read mod descriptions and see the requirements, any conflicts and/or compatiblity patches that can make them work together.
Also be sure to use https://nexusmods.com and not the Workshop.
Here is a good modding resource to get you started: http://steamcommunity.com/app/72850/discussions/0/594821545176386165/

Welcome to game and forums.

Order of plugins in your load order will determine which records will take effect in game. Records loaded after files above will win conflicts there.

Same thing applies to resources. If you have two files editing the water, then the latter in resource priority will take effect.

Resource priority comes in to play in two ways.

1. BSA files contain archived resource files. These are loaded according respective plugins in your load order.
   
   
2. Loose files shared outside of BSAs will physically overwrite each other over conflicts (but not official files in archives) and they will win all conflicts against resources in BSA files.

See pinned topics from main Skyrim forum here in Steam for more information. I am managing a pinned thread called Player spreadhseet. It should answer to any questions you have. Feel free to ask more in there or here.

Edit: Oh, I didn't see Mr. Sheffield's reply, while I was typing. :)

Thanks u guys a lot! :)

I made a list of every single mod I want to use, and now I'm downloading(from nexusmods website-thank nr Sheffield ;D ) them to check if they aren't aggresive towards each other