There is a lot of speculation as to the exact origin of the runes as a practical or mundane alphabet. What is known is that there are many inscriptions which have survived, particularly from Sweden, Norway and Denmark. There are a three widely accepted versions of the runes, the Elder Futhark which is composed of 24 runes, the Younger Futhark which consists of 16 runes, and the Anglo-Saxon Futhork which consists of up to 36 runes. As for who developed the runes as an alphabet, there are thoeries which suggest that the runes were derived from either Roman script, Northern Italian or Etruscan script, or perhaps the Phoenician script. In actuality in may be that they evolved from contact from all of these people, or may even have evolved by chance on its own.
What is known is that the alphabet, or Futhark evolved not only into a written language for practical and mundane purposes (and of course, grafitti) it also evolved into a magical and divinatory system. What remains of these practices can be found in historical acounts by Roman scholars, travelers to the lands where the runes were used, and even in the eddas and sagas of the Germanic peoples. These accounts allow us a glimpse into the religious and magical purpose of the runes.
In the Poetic Eddas there is a work titled The Havamal, or Sayings of the High One. In this edda we are told of how Odin took up the runes, screaming, from the well Urdabrunner as he hung from a tree-( the tree Yggdrasil, which roughly can be equated to the "World Tree" in Norse Cosmology) after piercing himself with a spear, being fed by no one, aided by no one. This mythos becomes very important as one works with the runes, as it describes a ritual of transformation and acquisition, one that becomes important as one begins working intimately with the runes on a regular basis- even if one does not work directly with Odin. I suggest reading the Havamal oneself, as it not only contains the telling of this in much better detail, it also gives insight into possible applications of the runes in a magical context.
It is very important to note that Odin aquired the runes from Urda's well, the well of the Norns which stands at the base of Yggdrasil.
There are a myriad of good resources about the history of the runes, both archeological and literary resourses that can be found both online and in your local library.