Even though Italy has a fairly extensive railway network, many smaller towns cannot be reached by train from Rome. The biggest company providing bus connections to towns outside the capital, especially to the northern part of the regione Lazio, is Co.Tra.L. Since it is virtually impossible to find parking space in the centre, the interlocal bus squares are mainly found near metro stations in the suburbs.
Interlocal buses Rome
Buses to smaller towns in the province of Rome itself, and the provinces of Viterbo and Rieti are usually run by Co.Tra.L. These can leave from different stations, but the biggest hub is the Saxa Rubra station. To get to Saxa Rubra you take metro A to Flaminio and then the train to Prima Porta.
Another big hub is the Piazzale Tiburtina at the Tiburtina railway station (metro line B, 4 stops from Termini). Here you can find buses to the Marche and to Abruzzo. (There are also buses to the Marche across from the Castro Pretorio underground station near Termini.)
Since the railway connection to Siena is not the easiest one, you could consider taking a bus there as well. These also leave from the Piazzale Tiburtina.
Other bigger hubs are to be found outside the Ponte Mammolo and Anagnina metro stations. The first one is very useful for day trips to Tivoli and the second one for the cities that comprise the so-called Castelli Romani, including Frascati and Castel Gandolfo.
Flixbus
The company Flixbus also has its home base at the Piazzale Tiburtina. This company is more of a cheap alternative for the train connections to bigger towns like Naples, Pisa and Florence.