Important to me. [Ubuntu Help]

I'm in a cyber defense team, we are doing our first competition tomorrow night. This will be a lot of learning for me because it is my first one. There are 6 people in my team, including me. I know the most about Ubuntu and I was given head of that, my brother is taking over of Windows for it. It is just Ubuntu, that is it. I need to learn what to do to stop anything from getting into the computer and stop anything that is on the computer.

We don't know what is wrong, troubleshooting will be done.
I get about 6 hours, they are run in virtual machines using VMWare
It is Ubuntu 12.something
If I could, learn about the Windows 7 one a little bit, if they need help.
I need to learn, It means a lot to me, it could mean a job, which is big for me. I could even go to collage.
Any help is nice. I run Ubuntu Gnome on my computer, along side Windows 8.1, I can run VM's.
Even if you don't respond by tomorrow that is fine. There are 2 other ones.

Since I assume that its past time how did it go? I kinda wish I was on the forums yesterday, I could have guided you on that.

There are still two more. This was my first one and I never got to do practice rounds. I was securing Ubuntu and I got a 62/100, I did better then the multiple people doing Windows. So I'm feelin' good about it. If you have anything to help now, anything counts. Either Windows or Ubuntu.

What did you do? Just curious.

Simple things, put on a firewall, delete things from an account. Changed account settings, remove permissions. Very simple things.