You can easily get to Porvoo by bus from many directions. From Helsinki there is frequent bus service daily. From the centre of Helsinki the bus trip to Porvoo takes about 50 minutes by the motorway, and from the direction of Lahti, Tampere, Kouvola or Kotka it's not a long trip either.
Timetables and ticket prices: www.matkahuolto.info
Local bus services: www.porvoonliikenne.fi