Well first of all, make sure you have an FTP software like Filezilla so you can transfer the server files over to ubuntu server. If you want, make sure you have a SSH program like PuTTY in order to run commands from your desktop computer rather than the server computer. For Java, (I can't remember what command I used to install Java since it's been half a year since I did that on my server) you would type sudo apt-get install openjdk-7-jre-headless. For the Tekkit part, download the server files, extract them into a folder on your desktop or someplace you can access easily, and connect to the server via FTP (I pretty sure that you also have to port forward your router for port 22 since FTP and SSH would be using that port and also 25565 for Minecraft) and then drag all of the extracted files into a directory in the server. From there you can right click the server.properties file and edit it as well as the launch.sh. Now in order to run the server, you have to type this command in(this is how I would do it if I had the server files in directories): cd ~/Minecraft/Tekkit. And to run it, I would then type sh launch.sh and let it run. For plugins, the only way I know how to do it is to use MCPC+ and then you should be able to use plugins. Sorry if this sounds confusing since this is the best that I can explain setting up a server on Linux.