Bangkok features a good number of main, long-distance bus terminals. Mo Chit Bus Terminal is the largest and busiest since it services Northern and North-eastern Thailand. From here, there are also (limited) services to other parts of Thailand. To get to Mo Chit you need to catch the BTS to the end of the line at Mor Chit/Chatuchak Market and then take a 10-minute taxi ride to the terminal.
Don’t be tempted to walk from the BTS station to the bus station as it’s a nasty trek for all but the most foolhardy! Buses leave frequently throughout the day, travelling to a whole host of destinations, including to Ayutthaya and Kanchanaburi.
Long distance buses tend to be extremely busy during public holidays as the residents of Bangkok rush to their hometowns to visit friends and family. It is a good idea to find out if any major public holidays occur during your trip and book transportation well in advance, or avoid travelling on these days.