I will be visiting Indonesia, Malaysia, and Singapore. I understand I need light clothing for the climate, but what I mean is should I avoid certain types of clothing, like shorts or tank tops? Are these countries very conservative? I don't want to offend but I would like to wear comfortable clothing for the weather, too. - Limey - Guest
Southeast Asian countries generally have conservative cultures, but you don't have to cover up and endure the humid weather under layers of clothing all the time. Just practice modesty whenever going out in public, i.e., don't wear shorts and skirts that are too skimpy.
Wear appropriate clothing when visiting temples and other places of worship—this cannot be stressed enough—and wear sheer or light cover-ups over your swim suit to avoid showing too much skin in public beaches.
Lastly, pack light! This is Asia, you will definitely find a hard time resisting the urge to shop for bargain clothing from the night markets! Take a hint from what the locals are wearing and fill your wardrobe with cheap (but high quality) and very fashionable clothing.
I am assuming you are a girl? You might find this very useful:
What Women Tourists Should Wear Around Asia (click)
Ingrid has some very useful and sensible tips, including opting for breathable sportswear / running shirts with sleeves, as well as convertible pants.
Glaiza is right about the highlands; they do get pretty chilly at night. Expect around 5 to 10 degrees Celsius. You will definitely need a good jacket.