Welcome to AA!
Even though you brought water home from your LFS, it really won't count for anything. All the beneficial bacteria that make your tank stable reside in your LR and sand. When starting a new tank, getting the water and LR in the tank is step one. Step two is initiating a bacteria cycle by introducing an ammonia source to the tank for 2-4 weeks. This will allow the bacteria in your tank to grow and break down ammonia and nitrites and enable you to get your first fish around week 4or so. It might be faster if your LR is already cured.
You'll know the cycle has happened by taking daily ammonia readings.You'll see the ammonia spike up, then slowly go down. About 3-5 days after the ammonia spike starts, you'll get a nitrite spike. Then as the weeks go on and your bacteria grow, the numbers will reach 0. When that happens your tank is now cycled and once you do a PWC of about 20-30% is ready for its first tenants.
During this time I wouldn't buy any livestock for your tank, no clean up crew, nothing. The ammonia and nitrite spikes will kill them, or at the very least shorten their lifespan by about 90%.
