So a T12 fixture can be converted to a T8 fixture pretty easy. All you have to do is change the ballast.
There are three kinds of electronic ballast for fluorescent tube lights (I can not make the link work for some reason, look up electronic ballast on wikipedia for more info) - you will be using an Instant Start ballast, the fixture will be left on for long periods with few on off cycles.
Unplug your T12 fixture, take the bulbs out, and open the case. The black thing with the wires coming out is the ballast. You simply cut the wires, take the old ballast out, put in the new ballast with the same screws, then strip and connect the wires with wire nuts. The ballast will have a wiring diagram on it. If it looks to complicated when you open the fixture let me know and I can take some pictures to post.
Again - I can not cut and paste a link - weird. goto 1000bulbs.com and look up part number BG-GE232MVN. This is an example of the right instant start (2) lamp ballast for you. A good hardware store may have them, or an electrical supply place will have them.
Like I said, if this kind a project is outside you're realm of experience I can post pics. I have an old fixture I can update to show you. It might sound complicated if you have not done it, but everything is color coded in the fixture - easy piesy.
