That depends on how you define "text". Do you mean any graphemic encoding of information, any encoding of information that can be converted directly* to speech patterns given the correct knowledge, or the oldest encoding of natural language that we could read as sentences? The oldest cuneiform tablets that we have date back to c. 3200 BCE, although the dating is complicated because they were mostly found in secondary contexts. However, it's not clear that they actually encode speech patterns because they don't really have extended texts-rather have groups of signs in boxes that we can work out from later texts are records of rations given out by the temple. Almost as old are the so-called "Archaic Professions Lists", which are exactly what they sound like-long list of profession titles that were probably created as exercises for trainee scribes to familiarize themselves with the titles they would need to know. Now these texts aren't really what we would call "literary" or something you would sit down and read through for fun; rather they were created like proto-cuneiform as a whole for primarily administrative purposes-recording who was supposed to get what when. Our oldest extant "literary" text is in all probability the Kesh Temple Hymn, an extended praise of a temple in the city of Kesh that describes its greatness and majesty. In fact, you can read it yourself in translation here:
http://etcsl.orinst.ox.ac.uk/section4/tr4802.htm
*That is, sign A refers to sound or set of sound B, etc-what is called "glottographic" writing.
I'm not sure about the oldest inscription, but I do know about the oldest literature.
The earliest literature that we know of is from the Sumerians, a Bronze Age Mesopotamian civilization. There was a city in Sumer called Abu Salabikh (not the name used at the time) that was excavated in the 1960's by an American team of archaeologists. Here they found a cache of 500 cuneiform tablets which contained, amongst other later documents, the Instructions of Shuruppak and the Kesh Temple Hymn. These have both been dated back to c.2600BCE, making them the oldest known pieces of literature.