It would help to know what your coop looks like. Size, layout, elevated or not, relationship of nests to roosts. Photos are really nice.
How do I know when they are ready to lay eggs?
You won't. There are a lot of signs they might be ready to start, an important one is that the combs and wattles will get bright red. But those are just signs that hey might be ready to start soon. They only real way to know when they will start is when you see an egg.
I have had pullets lay their first egg at 16 weeks. Not often but it has happened a couple of times. I've also had a group of three pullets lay their first egg at nine months. Not only did it take that long for the first to lay an egg but she started the first week of December, the shortest days of the year. She broke all the rules. Then another one started a couple of days later. With Leghorns and Buff Orps I'd expect an egg or two around five months of age but I'm not giving any guarantees.
Will they automatically go to the nesting box?
Maybe but not guaranteed. You can encourage them to use the nests by having a fake egg in them, I use a golf ball but many use ceramic, stone, or even plastic eggs.
It also helps to have the nests open and available before they start to lay. The actual egg laying process is pretty complicated. It's not just putting the egg together correctly in her internal egg making factory but knowing when and where to lay. Some pullets don't have good control of that process. That's why you can get so many weird pullet eggs when they just start out. Many have control of when and where they lay but some don't. About a week before they start many look for a safe place to make a nest. They will search out places that look good. That often includes scratching to see if they can arrange bedding to suit themselves. That's actually another sign that they may be getting ready to lay, if you see them scratching around in the nests. So having the nests open and with a fake egg in it is a good thing.
But some don't have that control initially. They may drop their first eggs at any time and any place. Maybe just walking around in the coop or run, or dropping an egg from the roost at night. Typically they gain this control pretty quickly, either in how to put the egg together properly or when and where to lay it. It may be a couple of days or it may be a couple of weeks. As complicated as the process is it's surprising to me that so many get it right to start with. And many do get all of it right.
If you see an egg in the same place every time, either in the coop or in the run, that probably means she has decided that is a good pace to lay an egg. If that happens get back with us and maybe we can help you retrain her.
What should I include in the hen house besides 1 roost and 2 nest boxes.
I don't know what yours looks like. You need access to all parts in there for different reasons. We typically have what we call a pop door that the chickens can use to go in and out of the coop. These are typically fairly small since a large door like a people door can let in a lot of rain and wind. Are you feeding and/or watering inside or out? They (and you if it is a walk-in) need enough light to see what you are doing. That could be a real window or it might be an opening covered with hardware cloth for ventilation as well as light. You need adequate ventilation. You are in Dallas, thanks for including that information. You are not really worried about cold weather but heat is dangerous. They will need shade when they are outside and you need to make sure your coop or nests don't become an oven in the hot sun.
If we knew what it looked like inside and out it might be easier to make specific suggestions.
