Craigslist! In Portland you'll have TONS of craigslist choices, and since this is your first bike and you aren't exactly sure what you will like best, if you choose wrong or your preferences change, you will likely be able to resell a used bike for close to what you paid for it.
For a 6 mile commute, I think the cheapest good option would be an older mountain bike (ideally no suspension), but switch the knobby tires for slicks. Old mountain bikes are durable, widely available used and cheap. They are also a great option in portland, cause vintage road bikes are super cool and everyone hikes up the used prices, but not on old mountain bikes!
At 6'1", finding a tall enough bike may be the most limiting factor for you. I'd look at some online bike fitting guides to get an idea what frame size would fit your inseam, then search CL for that size. When you see bikes that look promising, look up the brand and model on google to find out a bit about it. Make sure it's a bike shop brand (not a big box brand). If you can find one that already has fenders, lights, and a rack, that will save money over needing to buy those seperately.
Also, I'm pretty sure Portland has a few community nonprofit cycle shops you might check out. These places typically sell refurbished affordable older bikes. That might be a good way to get some advice and fitting locally without paying for a new bike.