Sounds like you're all set to do some good work--great!
As far as what you can drink... you need to make a decision. You can do a mono-juice fast, like The Master Cleanse where all you drink is one type of juice (the famous lemonade in case of the MC)... The Master-Cleanse is very supportive and allows you to maintain your daily schedule. I even did my gym routine every day of a very long Master-Cleanse with no problems. All the information about how to do this can be found on the Master Cleanse Support Forum .
Or... you can do a juice fast, with a variety of fresh fruit/vegetable juices. This is more work, since you have to shop, prep, juice and store freshly pressed juices every day. However, the juice fast is very supportive, you have lots of energy and it is very healing and cleansing.
Then the Big Mama, water fasting. I don't believe you should fast on water unless you are already pretty clean, or you risk some pretty nasty detox reactions. The recommendation for Water Fasting is pretty much restful conditions, no stress. In other words, you don't do it and go to work and the gym, as you can on juices.
As weight loss is your goal, I would recommend the MC. It gives you minimal calories, with maximum energy. If this interests you, go to the Master-Cleanse forum and read up, particularly the FAQs. You can also google The Master Cleanser , by Stanley Burroughs, and read the book online I believe, for all the facts before you get started.
So... sounds like you have some research to do before you get going! Whatever choice you make, I highly recommend you do a pre-cleanse diet, eating only fruits and vegetables for 3-5 days before beginning, and tapering off caffeine if you drink any! Going cold turkey on caffeine will give you the worst headache of your life. Pre-cleansing will make the first few days of your fast, traditionally the worst, as easy as can be.