And where did most of the wood come from to build their ships?

The Dutch already had one of the world's largest navies even before the Dutch Revolt. This was due to the Low Countries's position as one of the world's leading trade powers: by the mid 1500s Dutch trade fleets could carry more cargo than the Genoans and Ventians for example. Many of these ships were used in the war effort.

Furthermore, since the Low Countries was the richest part of the Spanish Empire, their revolt nearly bankrupted Spain, making it difficult for them in turn to raise land and naval forces to defeat the rebels. The Dutch on the other hand were able to use that wealth to help pay for the war effort.

The Dutch also had help from allies, first the French and later the British. The British had their own conflict with Spain, which resulted in the famous destruction of the Spanish Armada in 1588.

Finally, the Eighty Years War was not a constant conflict: there were long periods of relative peace in between periods of more intense conflict.

If you're interested in reading more you can consult:

Israel, J. 1998. The Dutch Republic: Its Rise, Greatness and Fall 1477-1806,

or for a more approachable book try:

Prak, M. 2005. The Dutch Republic in the Seventeenth Century.