There is no airport and the nearest train station is in Phitsanulok, about 80 km from Tak. Buses leave Bangkok's Mo Chit or northern terminal regularly and shouldn't cost more than 300 baht for an air conditioned bus. It will take about 7 hours but allow for those unforeseen hiccups and expect lots of stops at shops, etc. Once you get there if you want to walk to the town centre, it's about a 10 minute walk, about 20 minutes to the Viang Tak, get a samlor so as to avoid Tak's aggressive (at night) feral dog population.