Ranong can easily be reached by bus from Bangkok, Surat Thani, Chumphon, and centres further south such as Phuket and Khao Lak.
From Bangkok, both government and private VIP buses leave from the Sai Tai Mai (Southern Bus Terminal), with a journey time of about 9 hours. The bus ticket costs depend on the company and type of bus, and they normally depart between 19:00 and 20:00. Daytime buses leave around 09:00 but arrive in Ranong too late to cross into Myanmar on the same day. You can stay overnight in Ranong at a good local hotel in the city. Ask the local drivers (taxi, tuk-tuk) to bring you there.
While the cheaper but less comfortable second-class air-con buses are not recommended, the VIP buses are the most comfortable and offer the most legroom. Please check the exact schedule to get the correct times.
Here you can find timetables and tickets: 12go or Book A Way