If you want to be one of the first ones in the queue to get the iPhone 6, first you have to wait for Apple to announce the device. Only then will you know the actual sale date.
Generally, you'll need to line up early at either Apple or carrier stores. Otherwise, if you miss out on the first batch, you'll need to just keep checking back, since shipments do trickle into most stores on a daily basis. The main issue is that they might not have the capacity, color, or carrier model you want.
If you don't want to lineup, then you'll need to log onto Apple's site (usually midnight Pacific Time) and get in early. Suggestions I've seen include logging into your Apple ID before the site goes live at midnight and making sure that the billing address and credit card info are current.
Last year, the site went live a few minutes late, I completed the entire transaction in less than 5 minutes. Ordered the phone Monday morning, and it arrived on Thursday.
After placing the order, I checked back about an hour later, and all of the shipping dates were getting pushed back for the T-Mobile unlocked models, but still readily available in space gray on AT&T and Verizon. By the next morning, all of the models were getting pushed back. But, the ones reporting the longest delays (i.e., shipping in "October") were generally the 64GB models, the unlocked T-Mobile models, and the gold colored phones.
It seems that if you go with the black color and lower capacity, you'll get it sooner. Last year, the gold colored phones got pushed into November for some who ordered on the first day.
New iPhones generate a lot of demand, but it really varies.
