If you want buttons on your jukebox to control things you most likely would want to go with a keyboard encoder which is a hardware board which translates a button press into a specifically mapped keyboard key. Search for IPAC and KeyWiz for a couple of ideas.
If you get a touchscreen monitor then you would not need to have any buttons to control the software so you would not need to get a keyboard encoder since you would simply be using your finger to make selections, etc. I have an ELO Nec 17" touchscreen monitor and it works great for me. After hooking up the monitor you will need to install the ELO touchscreen drivers which you can get from ELO's website. There are other touchscreen monitors existing, but I have never tried others.
If you decided to install a trackball and two buttons representing right/left mouse click buttons (or simply use a real mouse) then you most likely do not need a keyboard encoder or a touchscreen monitor at all.
The minimum requirements for a jukebox computer depends on what you are using it for. My jukebox plays videos as well as audio files so my computer had to be fast enough to play videos smoothly (ie: without jumping through the video) and have enough space on the harddrive to store video files as well. Are you going to back up all of your music files once it is all setup properly to run on the jukebox software you selected? If so, then an additional harddrive might be required to be purchased.
Look through the jukebox software available (read the jukebox wiki to see a list and links) an try the ones which appeal to you. I would recommend looking at screen shots of the software to get a feel for the software which looks good, then read up about the software to determine which features the software has in it which you might want to be supported (dual monitor support, remote internet connection, kareoke, scrolling lyrics, etc.. etc ...). Then look at price and determine if you are willing to pay for software or if you want to use free software. Determine if the software should run on Windows, Linux, DOS, etc...
After you build the cabinet (or whatever) for your jukebox then installing the software and setting it up is pretty much simply placing the files in a supported directory format used by the software you have chosen. Also, you might need to update your audio files to have ID3 Tag information embedded into each audio file if the software required this as well. If you read the software's description you should be able to understand what would be required to set up the software for use. You can even simply set up 10 songs only in the required format just to test the software out first to decide which software you ultimately will use.
Hope this helps a bit.