Light cotton clothing, including a hat. Just plenty of cotton clothing - T shirts, shorts, and long trousers and long sleeved shirts (lots of biting insects) Don't forget to take a light weight waterproof - it really does rain a lot and it's not rain as we know it - more like someone has emptied a bucket over your head. Yes you will need good waterproof walking boots with cotton socks.
I have been to Borneo but haven't seen the orang utans - so lucky you.
The people are really friendly and the accommodation is extremely cheap and of very good standard. Be prepared to eat lots of chicken and rice.