What materials can be used for the top of a Sukkah?
Anything that grows from the ground and is not 'mekabel tumah' is a candidate for scach[sic].
These are two of the three basic requirements. The last is that the material not be currently attached to the ground. This disqualifies, for example, building a suka next to a vineyard and stretching some vines (which fill the other two criteria) over as the covering.
Trees branches and bamboo poles are popular. I most prefer evergreen tree branches because they look the prettiest and may even have a nice aroma. Anything that grows from the ground and is not 'mekabel tumah' is a candidate for scach. Fruit is an example of an item that is 'mekabel tumah.'
mekabel tumah = something that can become ritually impure
To tie the schach in place, many people/places use plastic cable ties, as they're not "mekabel tumah." Some rabbis consider this not the preferred way, as the tie-downs should ideally also be plant matter.