As Linux sysadmin, you will require to do a lot of repetitive and soon to be boring things like:
1. create/delete user, and reset password
2. change configuration files
3. update systems
Most of this tasks you can automate it via scripts or some configuration management systems such as puppet, chef, ansible and etc. But before you become Puppet Master, Chef, or etc, you will first need to do some basic scripting.
So, what you should start leaning is BASH and Python. If you got time, pick up ruby or some java as well (but is not essential).
You can learn BASH from the following links:
As for Python, you might want to start with:
1. http://openbookproject.net/thinkcs/python/english2e/ (or get PDF version here)
And most important, you need to get your hands dirty and start doing some scripts in BASH or Python.
Have fun scripting 😀