To get to Ostia Antica from Roma Termini, you would first start out leaving the Termini station, which is right at the Piazza dei Cinquecento. Walking out of the station you go to the left. Walking straight on the road, to the right of you and forward, you will see a blue sign that has the letter “M” on it. Walk down the stairs, and make sure that you follow the blue sign that reads “Metro Linea B”. When you get down the stairs, there will be a little room, and a machine named “Biglietteria”.
To buy your ticket for the metro, go up to the machine, and hit the touch screen on the British flag. There will be four types of tickets that you could choose from, and pick the first option. It will cost 1 euro for a one-way/75 minute ticket. I suggest you purchase 2 tickets, so that you have a return ticket for your trip back to Rome. Go through the turnstiles. Make sure you get through the turnstile for “Metro Linea B” going to “Laurentina”. Get onto the metro, and remain on until it stops at "Laurentina".
You will get off at the 8th stop, named “EUR-Magliana”. When you get off the metro, follow signs directing you to the “Lido” train. To get there, you will have to go underground and to the left. Go upstairs at the entrance to the “Lido a Cristoforo Columbo” train. Get onto the train and take it to “Ostia Antica”. It will take about 30 minutes to get to this stop, and it will be the 5th stop.
Get off the train, and leaving the train station cross the street using the “Blue Sky-Bridge”. Walk down Via della Stazione di Ostia Antica. Follow this road down to the parking lot located on your left.
Ostia Antica is open Tuesday through Sunday, May through December, excluding religious holidays. It opens at 8:30am until 1 hour before sunset. The link provided would direct you to a page designed to help you find out more about what to expect when arriving at Ostia Antica.
You will approach a ticket office, and tickets can be purchased for 4 euro per person. After your ticket is purchased, you will be able to explore the ancient ruins and be able to experience Rome’s largest port town that is now a preserved city, much like Pompeii. To get the map of Ostia Antica, you can pay 2 euro at the ticket office to get a detailed guide of what to expect upon your trip. If you refer to the guidebook Eyewitness Travel ROME you will be able to look through all of the history of places throughout Rome.
If you have time, the closest beach to is along the same train line as you would take back to Rome, and should be easy to find how to get to. Lido di Ostia is a suggested beach to travel to. If you refer to Rick Steves’ ITALY 2008 you will be able to get additional travel hints or advice on how and where to travel throughout Italy.