You can download the source tarballs here.
The releases are signed with the gpg key (8DFF53E18F2ABC8D8F3C92237EE0FC4DCC014E3D), which you can find here or on any keyserver.
Fedora: dnf install libssh-devel
openSUSE: zypper install libssh-devel
Looking for the latest release for openSUSE? You can find it here.
Debian or Ubuntu: apt-get install libssh-dev
Looking for a PPA for stable releases on Ubuntu? Look here.
Use vcpkg from Microsoft:
vcpkg install libssh
git clone https://git.libssh.org/projects/libssh.git libssh
To start with git take a look at the Git Community Book.
You can browse it here.