When trekking always wear light and quick drying clothing. Some people prefer to wear long trousers and long-sleeve shirts to ward off insects, leeches and scratches. But, you will probably find your guide clad in shorts and T-shirt. Good hiking boots or walking shoes are a must, as are sun block, insect repellant, sunglasses and plenty of water. That way, someone will be alerted should you get lost.
Leeches give many people the horrors.
They may be unpleasant but they are harmless. Don't pull them off, just apply a burning cigarette end or salt and the leeches will drop off. That's one of the reasons why your guide wears shorts and not long trousers. He prefers to see the leeches so he can get rid of them quickly.