A friend of mine owns a small solar installation company, so I tend to pick his brain. Overall, the sooner you contact a solar company, the better off you'll be. Having panels installed shortly after the roof is done would be smart. There are people that learn to install panels themselves, but it didn't sound like you were asking about that.
There are various products used for mounting the rails and I suppose they've changed with time. When they were first starting out 15 years ago, they would have to screw through directly into each rafter, so someone would be sitting in the attic, watching to make sure the first row hit them all properly. The mounts they're using now have holes for 5 screws, three of which have to be used, and only one which needs to penetrate the rafter. This gives a little more room for error. If the solar company had someone there while the shingles were being removed, they could take some measurements directly for rafter location, making their job a little easier when it comes for installation.
Before contacting a company, look over your yearly electricity usage, and think about whether you plan to use more (buying electric car, convert to electric appliances, etc) or less in the future. There is a good website to get an idea of how much production you'll get with a particular setup----
https://pvwatts.nrel.gov You'll enter your zip code, roof orientation, panel wattage, etc. It will then tell you how much output to expect (both kwh and $), so you can play with the overall system size to figure out how many you might use. I don't know about Ohio, but for most states, it doesn't make sense to produce more than you use on an annual basis. Panels are somewhere in the 400+ watt range, so a 6kw system would need 14-15 panels. There are 500 watt panels, but they tend to simply to be physically larger for the increased output.
The biggest choice you'll need to make is whether to use a string inverter or micro inverters. The installation company may push one vs the other. There are plenty of good reasons why people have a preference and it will also depend on things like tree coverage, roof access, or cutoff regulations.
edited to add--- here's a decent site to learn more and price out panels/mounts/inverters/etc--
https://www.altestore.com/store/inverters/micro-inverters-c1249/ I like looking here because it's educational and transparent. You can find better prices elsewhere, but its great for comparisons.
