Welcome to the forum!
My experience with losing weight has been that you need to make small, gradual changes that you can sustain- and then keep making changes slowly.
The most important thing to change is your food- without any other changes you'll see weight loss if you decrease your calorie intake. Have a look at your diet and see whether there's food you can safely cut out, or change for something that's lower calorie. Don't try to change everything at once- but make gradual healthier choices.
As a very rough guide, in temperate-cool climates drink 1.5 litres of water a day, and in warm climates at least 2 litres a day- but listen to your body. (And, as Omega said, the colour of your urine is also a good guide) It's certainly possible to drink too much- I find myself getting uncomfortable over four litres.
For walking (and exercise in general), do what you're comfortable with, or just a little more so that you build your endurance, and build it up gradually. If that's from one end of your house to the other, then do that. If it's a mile, then do that. That's something only you can judge.