If you are sure of your dates and time you can purchase your tickets now if you like. Purchasing in advance can save some money.
This discounted tickets may sell out. If you wait til close to your date you will probably be paying Base Rate- no discount- that price never changes
There are various restrictions on all different kinds of ticket-so you have to read the info provided on each offering.
When you pick your reserved seats (required on all fast trains) you will see the seating diagram that shows which way the seats are facing. If there is a table then the 2 seats on either side face the table and each other.
We've never found the need for first class or business class. That journey is less than 2 hours. The trains are clean, comfortable and the standard seats have always been fine for us. YMMV
Station names:
Napoli Centrale
Roma Termini
Official train site:
https://www.trenitalia.com/content/tcom/en.html
You might find this siteuseful:
https://www.seat61.com/train-travel-in-italy.htm