Rome to Pompeii and back

You can find organized day tours from Rome, but if you want to get there on your own, go to Roma Termini and buy a fast-train ticket to Napoli Centrale (trip takes about 1.25 hours). At Napoli Centrale, follow signage for Circumvesuviana (inside the station). Buy your ticket for this commuter train at the ticket window before you get to the turnstile, put your ticket into the reader and walk through - remember to retrieve your validated ticket, you may need to show it later (rare but possible). Go down the stairs to the platform for the train going to Sorrento, get off at Pompei Scavi (there will be postings of the stops in the train but they won't be announced) and you will be close to the entrance. Reverse the process to return to Rome.

Be vigilant about your belongings if the Circumvesuviana is crowded; if you are taking a backpack, hold it rather than wear it on your back. Keep nothing valuable in your pockets. If the train is not crowded, no worries.

Be sure the front of the train at the Napoli Circumvesuviana says "Sorrento" on the front. Some people have gotten on the wrong train (thanks to a local's advice!) and gone all the way around Vesuvius the wring direction. Not to mention names but her initials are LB.

If you know the day you will be taking the trip, the train from Rome to Naples and back has advance purchase tickets at a discount, provided they are not sold out.

The "base" ticket (full fare) is €43; the "economy" ticket (exchangeable before travel as long as you pay the difference, but not refundable) is €29; and the "super economy" ticket (no exchanges or refunds) is €19. That's each way, per person.

There is no advance purchase on the Circumvesuviana, and the tickets are cheap.

You didn't mention Vesuvius, which I consider missable. But many posters who make daytrips to Pompeii also want to swelter up the mountainside and scrape their knees on pumice stones, after baking on the stone streets of Pompeii. So note that Pompeii and Vesuvius are not located in precisely the same place - if you're planning on more than just Pompeii. FYI. Many travelers also hope to include the Pompeii frescoes that have been removed to the Archaeology Museum in Naples. But this makes for a challenging time schedule, although Naples has perfectly good local transit.

As I understand, once you get to Pompeii, there will be guides available to give us a tour. We do not need to book a tour in advance.
Is this correct?

How do we get the advance sale train tickets? Can we wait until we get to Rome (9/26/15) or can we get them months in advance before we leave for our tour?

Yes, there are guides onsite at Pompeii.

You buy you tickets in advance on the trenitalia.com web site. They may ask for an Italian address to complete the reservation. It means nothing so just us your hotels address. All tickets are sent to you via email.

Trenitalia tickets go on sale 120 days in advance. The cheapest tickets sell out first, but the full fare ("base fare") tickets almost never sell out.

If you want discounts and can accept the restrictions, book as soon as you know your dates. If you want full flexibility, you can wait until you get to Italy to buy your tickets (as long as you're OK with paying base fare, it doesn't confer any saving to buy in advance). However, if you want to leave Rome on an early train, buy your tickets at least the day before, as the lines can be long at the windows, and sometimes at the machines.

I'm trying to find that fast train from Termini to Napoli Centrale on the Trenitalia site. The fastest one I can find is nearly 2.5 hours.

Any suggestions?

What date are you looking at? I pulled up a random June date and see several at 1 hr and 10 minutes.