Now that I'm done being a smart @ss... sound like you did a no-sparge mash and didn't properly account for water volumes. As already mentioned, you need to account for grain absorption, boil-off, other kettle losses, loss to trub, etc. to get the final amount of beer that you're after. Brewing calculators or software like BrewTarget or BeerSmith can help.
Adding water pre-boil diluted your wort (The juice is called wort, BTW). A batch sparge (adding more water to the grain and draining again) would have been better for reaching a proper pre-boil volume. You'll wash more sugars from the grain and not just dilute with plain water.
May I recommend "How to Brew" by John Palmer?
http://www.howtobrew.com/
The 1st edition is online for free. While some stuff has changed since the 1st edition came out, you'll get better responses if you get some terminology down, so that folks know what you talking about.
Look into getting a hydrometer, as well. You'll want to measure your pre- and post-boil specific gravities (SG) to help you determine if things are going as planned