There is no need for vaccinations against yellow fever, hepatitis, or diphtheria and antimalarials are not necessary if you only visit the tourist areas in Thailand. Like Bangkok, Chiang Mai, Pattaya, Phuket, and other popular holiday destinations.
However, you may need one or more vaccines if you plan to travel off the beaten track or visit neighbouring countries, like if you go trekking in the jungle near the border to Myanmar or if you want to cruise the Mekong to Luang Prabang in Laos.
Your government may have an office informing you about the necessity of vaccines in various parts of the world, like TravelHealthPro which is set up by Public Health England.
Contact your doctor if in doubt. In addition, you can find vaccination services on the internet; however, they may be more prone to sell you one or more vaccines, even if you do not need them.
Even if you do not need to get vaccinations, you should practice mosquito bite avoidance, for instance by using insect repellents and covering up with clothing. The most effective repellents are DEET-based; however, you should not use DEET based repellent for infants below two months of age. Read more at fitfortravel. You can buy insect repellents everywhere in Thailand.