Minecraft Forge is the best way to mod your little world of diamonds and Endermen (Endermans?). It's designed to be easy-to-use, and is the go-to place for all your Minecraft modding wants and needs. Unfortunately, this doesn't actually work with all PC versions of the game. Minecraft Forge works with the Java edition of the game, but it doesn't work if you've got the Windows version. If you're looking to get Minecraft for the mods, then make sure you pick up the Java version.
Now that you know all of that, it's time to get down to business.
How to install Minecraft Forge
Installing Minecraft Forge is actually pretty simple. First, head to Minecraftforge and download the program. You want to make sure that you download the version of Forge that corresponds to the version of Minecraft you have. So, if Minecraft is 1.1, you download the 1.1 version of Forge.
Next, go to wherever the file downloaded - it should be 'C:\Users\\[username]\AppData\Roaming\\.minecraft\mods' and open it up. This will bring up the installation process. Make sure you leave 'Install Client' selected and then hit 'OK'.
The next time you open up the Minecraft launcher change the profile to 'forge'. Once the game loads, you'll be able to see a new 'Mods' menu on the start screen. This is where you can choose which mods to use.
That's it, now you know how to install Minecraft Forge, and from there it's just a simple matter of finding the Minecraft mods you like and installing them. There are more blocky guides where that comes from, so here's how to build a house in Minecraft. Now you've got a home for all your handy mods, and for yourself.