Hi Marion,
If you look at the holidays on the website you'll be able to make a comparison. From memory, I think your holiday starts in Thailand, in Bangkok and you travel to Cambodia overland? We started in Hanoi in northern Vietnam and made our way south. I think the places in common are Ho Chi Minh City (formerly Saigon), the Mekong Delta, Phnom Penh and Siem Reap (for Angkor Wat). There's no need to be nervous - the tour managers are all excellent and they will make sure you are fine. Even though these places were new to me, they were not intimidating in the least. The people are very friendly and JY will take you from place to place, so you don't have to find your own way anywhere or have any language problems - the hotel staff speak a little English, although service can be slow!
Regarding clothes, I think the time you go it will be very hot and humid everywhere (but do a google search for Cambodia climate etc just to be sure). So lightweight clothes are the way to go. It was even hot in the evenings, up to 30c! Hope you have a brilliant holiday and enjoy every minute of it. Although my holiday had quite a different itinerary, if you have a look at my review on the "Reviews" section, you might get some helpful info regarding the bits of the holiday that are in common with yours.
All the best,
Bob