First of all. The most important thing is water. This is the most important shit ever. ever ever. ever.
Cut out all drinks except water. Anything that isn't water is a diuretic and your body has to work to break it down, thus dehydrating you.
You need a minimum of 8 glasses of water a day just to for your organs to run. 64 ounces. If you're 240, then make it 120 ounces.
But slowly start rehydrating. If you've only been drinking 2 glasses of water a day, then move to 4 or 5. Then eventually to 120 ounces.
Cut out all the junk food, no cheating.
No drinking anything else until you've gotten to make 120 ounces a daily habit, because that's bare minimum for your brain, organs, and everything to work properly.
Once you do this, then you'll be ready to start working out.
When you sweat working out, you'll need more water to replace that sweat.
Any time you drink anything besides water, you will have to replace that much water back into your body.