This is an easy way to download and install mods for Minecraft PC.
NOTE: This is an old method of modding, and I am quite frankly unsure if forge still exists. I have ceased to continue playing Minecraft, so I am unsure of the actual functionality of this. If forge no longer works, please drop a comment below so I can remove this instructable.
Step 1: Forge
Look up "forge download"
There will be a screen with a whole bunch of links titled the minecraft version, recommend, and latest. It's a lot to take in. But it isn't to hard to navigate the site. Simply click the latest link that says "client" which Matches YOUR version of minecraft. (this one is an example of 1.7.10)
You will now be directed to a page called ad fly it will show an ad. DO NOT CLICK THE AD, instead, look on the upper right hand corner of the site, and there will be a skip ad button. Go ahead and click that.
Once it downloads, open it and install
Step 2: Download Mods
Once you have forge installed into Minecraft (open Minecraft and change profile to forge, and play. If another button appears called "mods" then that means forge was installed correctly. If not, delete forge and reinstall) download any mods that you want that IS COMPATIBLE WITH YOUR VERSION OF MINECRAFT (should say 1.7.10 [per my example])
Step 3: Drag Mod File Into Mod Folder
Once you have the mods you want downloaded, you will have to manually "put the mod in minecraft." To do this, go to start menu or finder and search %appdata%. A file should appear called "roaming" Click on that and then click on ".minecraft" select "mods "(create a new folder called mods if there is no mods folder) and drag the mod you wish to add to minecraft in the mods folder.
Step 4: Open Minecraft
Open minecraft in the profile of "forge" and click mods. The mod you added should appear. If a problem appears, drop a comment and i'll help you.