Guilds are were mainly professional corporation that organised it's members and regulate a particular trade or area of expertise. So they were not unions as much as modern companies, but the analogy isn't too bad to understand some aspects of the guilds, as enforcing rules and privileges in trade, etc.

I dont know about the earliest (as in the first of the first) labour unions. The first syndicates and unions were largely informal and repressed by bosses and local powers. I guess it could range from 1650 to 1750 as the earliest examples of manufacture and organised work. The organisation of labor was explicitly banned in more than a few laws in various countries during the XVIIth. Of course, the real answer depends on many variables in your question.

Your question seems vague: of course workers organised to an extent since a long time, maybe since there's work. And again, what is a worker? Do your refer to strict members of a social class, such as the proletariat, or do you refer to the modern concept of work, or to anyone that ever did work on behalf of someone else? Or is any slave that spoke on the behalf of others to it's master might subscribe to your criteria?