To get out of Melbourne, take West Gate Bridge, follow M1, then follow signs to Geelong(Princes Freeway). This takes about an hour (70 kms south west of Melbourne). Once you reach Geelong, follow signs to Torquay (turn left at Torquay Rd) which is another 22 kilometres from Geelong.
BY Public trasport is a bit more tricky. I only know the train journey from Spencer Street Station (Melbourne). V- Line goes at least every hour. It takes a little more than an hour from Spencer St Station to Geelong Station. Costs $12.80 full one way fare. Plus at least $35 dollars taxi fare from Geelong station to Torquay. Don't know much about the bus timetable. I heard it's not very frequent.
Bus is the only form of public transport available, unless you want to pay $30 for a taxi from Geelong.
The bus leaves from the Geelong train station, makes several stops in Geelong before heading on to Torquay. It continues on to other popular coastal towns on the Great Ocean Road (Lorne etc).