Sorry, but becoming a shinigami is out of the question for multiple reasons. First, shinigami are just the Japanese interpretation of the European Grim Reaper. Second and more importantly, both creatures are a form of psychopomp, a spirit that guides souls to the afterlife. A being like that is not something a human becomes, at least until (assuming my knowledge of the afterlife is correct) after their death upon which they may be granted such an option, among others, in lieu of reincarnation.
To put it simply just focusing on one aspect of the shinigami myth reveals why it is impossible though becoming one would have nothing to do with altering your DNA as a shinigami wouldn't posses DNA as they have no physical form. Based on your profile picture I'm assuming this desire was inspired by some form of Japanese anime such as Bleach, but even in the anime shinigami or soul reapers are spiritual beings. In the end if shinigami, grim reapers, angels of death, etc... existed and if you could become one it would occur after your mortal death when you become a pure spiritual entity. As stated above shinigami and other similar beings within other cultures are spiritual guides that help people crossover into the corresponding afterlife they are linked to, so they would need to be spiritual beings to interact and guide earthbound spirits to the other side properly.
if you think such a concept really exist you are wrong my freind.. too much anime, try study the true magic some more... a lot in fact and then if you have some troubles understanding some of the real stuff we will try to guide you
The only reference I know is that Shinigami comes from an anime called Death Note. Yes I have written a spell to make a Death Note, but I never took it seriously enough to try it out. Even if you could the price for Shinigami Eyes was half of your remaining lifespan. If that is the given price for Shinigami powers, why would you want it? As for the Death Note itself, that power if used would surely draw attention if people start dying and the law enforcement begins to see that all the deaths surround a cirtain individual. Even if such power could be obtained, it is not something that should be attempted. If you're trying to get rid of an abusive uncle, there are safer ways to do it than asking for the power to kill.
I actually looked up what a shingami was. It is not something I want to mess with. Wikipedia's initial description. Here's the full article. http://en.wikipedia.org/wiki/Shinigami
In Japanese culture, shinigami (?, "god of death") is a god or supernatural spirit that invites humans towards death, or induces suicidal desires in humans.  The shinigami is a personification of death, but whether its personhood is literal or merely poetic anthropomorphism varies in Japanese religion, literature, and folk tradition. In modern Japanese fiction, the shinigami is most often literal, and synonymous with the Grim Reaper.