I play guitar ( though most who hear it would disagree) and I am self taught. If you know any guitarists, the quickest way is to get then to show you a few basics, from how to hold the guitar to a few basic chords.
You do not NEED to be able to read music to play guitar or keyboard as there are things called tabs, which are basically diagrams of where to put your fingers. The only problem is, that the tune has to be familiar to you with tabs, unlike sheet music. This may help you with understanding tabs and chord diagrams. Once you get the hand of it, it is pretty straightforward.
http://www.zentao.com/guitar/tab/
Here is an excellent site that will help you with finger positions for chords. I would suggest that you learn the chords. D,A,G at first then when you have learnt those you can strum a reasonable sounding tune.
http://chordfind.com/
Getting a guitarist to show you these chords will help a great deal. It will take a few minutes to show you but maybe quite a bit longer to work them out yourself.
Keyboards also have tabs and chord diagrams that work in a similar way showing finger positions.
A lot of the things you see first will seem alien to you but if you can get someone to show you that will help a great deal. Also very good instructional DVD's are available from most musical instrument shops or online.
Finally, a couple of pictures showing parts of a guitar.
http://www.learnthat.com/courses/fun/guitar/im ages/guitar1.jpg
http://www.geocities.com/Heartland/Pl