Do you want to go to Ostia-Lido (modern beach town) or Ostia-Antica (ruins of the ancient port town of Rome)?
The later is really worth to visit. You might compare it to Pompeji. It is a whole town with theatre, monuments, shops, villas, inns etc.
You can take public transportation: Metro B to Piramide (next to the railway station Roma Ostiense), Basilica San Paolo or Eur-Magliana, from there take the local train (Ferrovia Urbana Roma-Lido) to Ostia Antica. The names of the stations show you which other sights you might combine the excursion to Ostia with, most notable St. Paul's Cathedrale.
If anybody wants to go to the beach, the station to use would be Lido Centro or Stella Polare.