Suppose you drink 4 sodas/day - switch out one of them for water. Do this
for a week, then switch out another
for water; do that
for a week, the switch out another for water, until eventually, you don't drink sodas, but you drink a lot of water (you can also do unsweetened tea, coffee, etc... they all count as water.
So let's start with calculating your calorie needs and go from there.
Do you do any exercise? If so, how much, how often? What type?