Not being the first commercially available Character Sheets, those were done in 1975 by Wee Warriors for D&D and were called The Character Archaic, and it’s easy to find images of them online, these are the first official TSR produced and designed sheets. 

It’s interesting to see how close they are to our present character sheets, but also how different they are as well, from all different kinds of Saving Throws, to Psionic Strength info, many of these things have fallen by the wayside in the last 45 years. However, the central concept of a character sheet is pretty much unchanged as are the main 6 ability scores and concepts like HP, Saving Throws, AC (although it was reversed, with lower being better as it would be until the third edition in 2000 did away with THAC0) remaining until today. A very insightful couple of pages.