Okay - I'm a bit of a newbie - but for what it's worth - if all you've got is tap water, then you have NO CYA (cyanuric acid or stabilizer) so whatever chlorine you put in will not hang around for long. Based on going for a shock of 10ppm - you can start with 1 gal of Bleach but remember it's not going to last long with stabilizer and you need to keep it there until you kill all the algae. Get some CYA and/or some stabilized chlorine pucks and a floater. Got to www.poolsolutions.com and READ everything. That will at least get you started.

You also need to get a decent test kit - info about that at poolsolutions.com too. I'm sure you'll get more knowledgeable information if you just hang around for a while.