My 2 cents on some of the various advice:
Demo/ride a bike first: this depends, when I first got into road cycling I had no idea about - well, anything. Riding different bikes meant nothing to me. Even riding a cross vs road bike meant very little. This was a waste of my time - I simply didn't know what I was looking for. They all felt "fine." Which is to say that I wasn't particularly comfortable on any of them, but I had never really ridden before other than just riding as a kid and well all rode bikes all the time. Even with much more experience I don't think this would help me that much. It *might* help a bit with determining between road/cross/mountain and possibly material, but not much else. And not manufacturer.
Road vs cross vs mountain or in-between bikes: this may depend on what you really want to do, but in-between bikes are generally not great. Having said that, that's exactly what I got my wife and she likes hers. If you are primarily going to be on the road, get a road bike. Cross bikes seem like the ideal solution, but they are really meant for cross racing, and you probably are not going to do that. Mountain is great - for mountain biking. But they suck on the road. So this is more of a depends on what you are doing.
Aluminum vs Carbon vs Steel: All are great, and all have advantages disadvantages. I won't get into titanium b/c this is $$$, but it is great too. Aluminum will beat you up this most, but you still see pros riding aluminum (well, you would if sponsors would let them and non-protour guys). Aluminum is generally the best bang-for-the-buck. Carbon is great and light, but if you crack the frame you are pretty much out of luck and it is more pricey. Steel has the best ride of any of these (not just my opinion), but can be heavier. All can be expensive - a quality frame is a quality frame regardless of the material. Lighter is generally better, so get the lightest you can afford. And work on losing the heaviest component - you (gut, love-handles, etc.).
Groupsets: probably the most important thing, as someone earlier said if you can get Shamino 105 components, that's all you'll ever need. And if you take care of them, with a little maintenance you can hand them down to your kids - they'll last a very long time. Go for the most your budget will allow. Compare different bikes based on their groupsets more than the bike manufacturer.
Fit: unless you are seriously getting into triathlon, youtube should tell you all you need to know about fit. It is important, but until you are riding >25 miles it makes little difference. Even then, youtube is your friend. With tribikes it is a different story.
Brakes: for road, get rim brakes. Less hassle, and if you are riding something that really needs disc brakes - well, you aren't. Disc brakes are generally hated by most pro road cyclist, they just have to ride them because of bike sponsors ($$). That's for a slightly different reason, but I stand by this. I've had both and never felt the need for the heaver/more expensive/more maintenance brakes. For mountain it's a different story and this is much more important. I live in a very hilly area in Austin and my rim brakes work great.
General: Buy used. Craiglist, eBay, friends. Buy in the "offseason" if you can (can be tricky, end of summer probably best). If not used, last years model. Don't get into electronic shifting, it's not worth it and will eventually break or need replacement. Just get mechanical and it will last you a long time. Buy some decent wheels. They'll possibly cost more than your frame, but they are worth it if you want to go faster. But - if you want to go faster, ride faster. It is that simple - nothing will replace it. Buy inexpensive but quality shoes (expect to pay around $80). Buy the cheapest helmet you can find that (in Texas) has a lot of ventilation. Clothing - don't go overboard, just find something that breathes and has some pockets for spare tubes, etc. Cycling shorts - just get something that has some padding, but don't go overboard. Get you butt used to the uncomfortable saddle - it will break-in (your butt and the saddle). Pedals - this may depend on what else you would be doing, but I despise Spd pedals and this is a lot of what you'll find on lower-end bikes. Get some SPD-SL/Look or really anything else - just don't pay a lot (< $50: https://www.rei.com/product/710073/shimano-r540-spd-sl-road-pedals). Even just toe straps to begin with wouldn't be a bad option.