If you are dressed inappropriately you will be told about it and not allowed entrance. Casual clothing is acceptable for tourists, but when visiting special places do not wear short shorts, halter-tops or muscle shirts. Shoes should be comfortable and good for walking. Remember when entering someone's house, a temple and some places of business removing your shoes outside is required. High top boots with many laces are great for trekking through the jungle, but after having to remove them for entrance, you will quickly consider footwear which is easier to remove.
Additional clothing is easy and inexpensive to purchase in Thailand.
All kinds of shirts, slacks, shoes and accessories are readily available in the stores and on the streets of the cities in Thailand. Of course, souvenir T-shirts are available in all tourist destinations. Pack light when you come to Thailand, the low prices and selection of clothing available will happily surprise you. Even the tourist on the most modest budget will return home with some clothing purchased in Thailand. Additionally, laundry service is available in the hotels and resorts.