Well first of all you need to know what mods you want; most mods run on Forge Modloader, but you'll figure out how to install that on many of the mod pages or on Youtube. (Also this is the wrong section to be in not a BIG deal, but just so you know)
Basicaly what the title says. I want to know how to install mods in Minecraft 1.7.5
As said above, most mods use an MPI (Modification Programming Interface) called Forge (FML, ForgeModLoader, which is what Stormman09 was referring to, is the actual part of Forge which loads the mods into the game and provides the base hooks for mods to use, Forge is the rest of the MPI). You can find the downloads for Forge on their fileserver (files.minecraftforge.net), and it's pretty simple to install, literally you need to just download the installer and run it (if you haven't messed around with any of your Minecraft installation settings, you should be able to install it fine with the default settings).
Unfortunately, no Forge version is out for 1.7.5, the only versions out in the 1.7.X branch of Minecraft updates is 1.7.2 and previous, and 1.7.10. So you'll need to instead create a new profile under the 1.7.2 or 1.7.10 version. I'd recommend 1.7.2 as that's what most mods updated to 1.7 are on currently.
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Author of the Clarity, Serenity, Sapphire & Halcyon shader packs for Minecraft: Java Edition.