I want to lose weight?

I'm about 238. Is it reasonable to be around 200 before school starts next fall? I'm going to diet and drink ONLY water/milk, the only exception is when I go to the theater and get pop and popcorn, but I'll probably only see Narnia and Dark Knight. I'm going to get a membership to the gym too. Also, gas is expected to be 4.00 a gallon, and is already 3.59 here, so I'm going to walk or ride my bike everywhere, except to work because it's 20 miles away. If i keep up with this, can I be around 200 before school starts in Mid-August? also, feel free to give me ANY tips!