I don't want to worry you too much, though I hope this makes a difference in how you think about Mountain Dew. Not only is this soda associated with weight gain, but it is also correlated with serious tooth decay and dental problems that affect your overall health eventually. So, think about the pain and expense of dental work that you want to avoid when you are drinking your Dew.
I would not suggest cold turkey, but cutting back gradually and substituting something else.
Is it the sweetness you crave? The caffeine? Depending on what you are trying to wean yourself from, that will dictate your substitute.
If it's sweetness and caffeine, then try to make iced coffee with honey. At first substitute this for one can of Dew per day. After a week, substitute it for 2 cans, and so on, until you are done with the Dew.
Then if you wan't to kick the caffeine, slowly switch to decaf iced tea, etc.
Eventually, you want to make water your drink of choice, and you should start drinking a glass of water for every glass of other beverage you consume while you quit.
This is the best advice I have, but no doubt other Sparkers will chime in.
I wish you all the best. Remember, you are WORTH a healthier lifestyle, and YOU CAN DO THIS!!