Try to avoid spending your first 3 days in Bangkok. If you start here after your intercontinental flight, there is a big chance by the third day you are so overwhelmed by the heat, the noise and the cars of the city, that you just will want to get out! I know this is a convenient way of starting your holiday in Thailand, from the plane into the big city. You are there anyway. But better spend the first 3-4 days on the beach. Bangkok is a hectic crazy city. It is hot, humid, busy, never sleeps, noisy, smelly... Keep this experience for the final days of your stay as you will be more used to he temperatures and the humidity and you will have had some good nights sleep.
In general Thailand has hot weather. The seaside and the mountains can be cooler in some parts of the year. Now from north to south, you have a distance of roughly 1300km, with some ocean and some mountains. You know from home that the weather can be very different just a few 100 miles / kilometers away. Here it is no different. Thailand has a rainy season that is different depending on where in the county you are. I have made a chart to show you when you have he coolest temperatures and when you have the least rain.
I have been many times in Phuket, Bangkok and Pattaya during the month of September and my experience with rain season was not that bad. It rains several times a day for 15-30 minutes and is then dry again for a few hours. Some days will be 24 hours of rain others will be cloudless. Tha past years they even had serious floods in some parts. If the beach is your primal reason for traveling to Thailand: avoid this period. If you go for traveling around, sightseeing, shopping and relaxation, then this period might even be better for you. Prices are cheaper, people have more time and you can have a great holiday anyway.
When does it rain?
How hot is it?