I'm not a medical professional or anything and my kids are older now (4 and almost 3), but I remember giving my kiddos about 2 oz. of bottled water in a sippy cup at about 5 or 6 months when we first started learning how to use a cup. Tap water can be harsh and dangerous on little tummies, so use bottled or nursery water first (something about Nitrates).
The American Academy of Pediatrics has some good info. on their website. Here are a few highlights:
1 - breastfed exclusive babies don't need water supplemented
2 - formula feed babies - 2 oz as needed from 4-6 months - sippy cup preferred
3 - no juice before 6 months. Juice only in a cup, not bottle, only 3 oz per day, never exceed 6 oz. per day of 100% juice
4 - juice is directly related to short stature and obesity in children! (so water is better than juice, juice isn't even necessary!)
You can go to their website: www.aap.org
Another website I love is Dr. Sears: www.askdrsears.com
Best of luck mama! Enjoy your new little one! I love them at 4-6 months or so when they reach maximum chubbness! The have all those cute folds in their arms and legs - adorable. Then they start movin' and it all just melts away! :)