How to install Forge: For Players
Go to http://files.minecraftforge.net and select the minecraft version you wish to get forge for from the list.
You can download the installer for the Recommended Build or the Lastest build there. Latest builds may have newer features but may be more unstable as a result. The installer will attempt to install forge into your vanilla launcher environment, where you can then create a new profile using that version and play the game!
Here is a short video from Rorax showing how to install and setup Forge.
For support and user questions, visit http://www.minecraftforge.net.
How to install Forge: For Modders
If you wish to setup a new mod for Forge, visit http://files.minecraftforge.net and select the MDK download.
This is the Modder Developer Kit - basically, an example mod with all
the tooling needed to create a Forge mod workspace ready for use in your
IDE of choice.
Here is a short video from @cpw showing how to install and setup the MDK in Intellij.
How to install Forge: For those wishing to work on Forge itself
If you wish to actually inspect Forge, submit PRs or otherwise work
with Forge itself, you're in the right place! Clone this (either
directly, or make a Fork first if you want to make a PR) and run
gradlew.bat setupForge or gradlew setupForge. This will download and
setup all the bits you need to have a functional Forge workspace.
After that, open a workspace in your favorite IDE and import existing projects the projects folder.
Here is a short video from @cpw showing how to install and setup Forge in Intellij.
Pull requests
Pull requests should target the current default branch. Currently, that is the 1.10.x branch for Minecraft 1.10.2.
If you intend to modify a minecraft patch, or add a new one, always run
gradlew genPatches to generate your patch and validate that you
are not doing anything unexpected in your patch.
Contributor License Agreement
As stated in the (https://github.com/MinecraftForge/MinecraftForge/blob/1.10.x/LICENSE-new.txt) file, we require all contributors to acknowledge the Forge Contributor License Agreement. Please ensure you have a valid email address associated with your github account to do this. If you have previously signed it, you should be OK.