Workers are gained by building housing structures, such as tents and cottages.
Almost all buildings require workers. Units also require 1 worker each.
Strategy
Workers may be freed up by switching off production buildings preventing them from accruing further resources until their workforce returns.
Having negative Workers supply doesn't seem to inflict any penalty, and the downsides are only secondary (preventing you from training new units or building/repairing buildings that need workers).
Be careful of building workshops. They consume a good chunk of your workforce and you cannot shut them down, so if you pause a sawmill to build a wood workshop, you might inadvertently cause a crash in your colony's economy.