Just like Splint says, you're gonna want to re-evaluate your goals within a few weeks. Often times, you will see significant results while your body gets used to the significantly lower intake in calories, but once that happens, you'll want to re-evaluate.
Rome wasn't built in a day, likewise, losing 120 pounds in 6 months is not impossible to do, but very difficult. I don't mean to discourage you, but you'll understand once you're a few weeks into your new regimen.