QuantLib Git Repository
If you're new to Git, you can read the free Pro Git book; its first couple of chapters contain the information you need to get started.
Also, the commit guidelines in chapter 5.2 are suggested reading, especially the part on commit messages.
For the time being, the main repository is the one located on GitHub at https://github.com/lballabio/quantlib; from there, you can also browse the code and the commit history. If you only want to check out the code, you can clone it directly by issuing the command
git clone https://github.com/lballabio/quantlib.git
(if you're using some other tool, the actual steps might vary but the same URL can be used). However, and especially if you want to contribute, a better way is to get a GitHub account, fork the repository, and check out your fork to your computer. This way, you'll be able to push changes to your fork and ask for them to be pulled into the main repository.
Note for C++ Unix users: if you use Git, you will need some GNU tools that usually only developers use, and which are not required to build QuantLib from released tarballs. These are automake, autoconf, libtool, GNU m4, GNU make, and others which might escape me now. They all come with recent GNU/Linux distributions.
To begin the build process from a Git working copy, start with:
which will prepare the package for compilation. You can then use ./configure and make in the usual way.