If it is semi-managed then the provider should have all the software installed that you need, and should be able to help you with any configuration issues.
From there, I would recommend making sure that you have logwatch or something similar installed (again your provider can probably help you here) to email you each day with a summary of your server logs. This will help you identify problems such as floods of spam, breakin attempts, etc.
If you have no experience with logging in via SSH then you'll want to start learning how to use the Linux shell. The best way to learn Linux without screwing around with your VPS is to get an old computer or a free virtualization package like VmWare Server and install the same Linux distro locally that you have on your VPS. That way you can try out new things without fear of messing up your VPS box.
One of the first things you'll want to do is install a firewall. Many VPS users use APF, which is relatively simple to install and configure, but it requires you to know how to get around on your box with SSH. Your provider may install a firewall for you, I'm not sure.
Thanks. I have been informed that they have indeed installed enough software to get me going. I will ssh into the box and poke around to see what has been installed/configured. Thanks for taking the time reply. I'm sure i'll have many more questions. If there a good place to get general tips on running your own server?