Sometimes pirate crews started as mutineers. A merchant or navy ship has a captain that severely mistreats his crew. Eventually they get sick of him and maroon him somewhere or throw him overboard. At that point, everyone on that ship has probably already committed a capital offense (especially if it's a navy ship), so they literally have nothing to lose by becoming pirates at that point. If they're ever caught, they're already dead. Initially on the boat are just the folks who happened to be there at the time of the mutiny. But some filtering happens over time. Those who don't actually want to be pirates eventually slink off at one port stop or another. And new recruits are found via quiet conversations in port taverns. A lot of recruiting probably happens through personal contacts, "oh, we're going to port X? Why I have a cousin there that might be interested!"

Also, often pirate ships started off as privateers. A privateer is just like a pirate, except legal. Let's say Britain and France are at war. Britain issues letters of marque to any ship with captain and crew willing to raid French shipping. This is a way for the British crown to target French shipping at a fraction of the cost of having the Royal Navy do it themselves. (It's the Age of Sale equivalent of German U-boats raiding British shipping during WW2.) If such a privateer is caught by the French, they'll still be treated like pirates. But as far as the British are concerned, what they're doing is completely legal. They can go steal a French and ship then sail into any British port and unload their plunder. It's all completely legal and a culturally accepted part of warfare. As long as the captain had their official letter of marque, the harbormaster in any British port would let them participate freely and openly. As far as how these privateers got going? Simple capitalism. Some people with cash to invest would invest in a boat and crew, and attempt to make a profit from the conflict.

And that's all well and good, except what happens when the war ends? Those letters of marque were only good during wartime. And privateers don't get military retirement benefits. There's no pension or these folks. (Not even whatever pitiful equivalent existed for Navy sailors at the time.) Men who were risking their lives sailing the seas, plundering commerce in Britain's name? Well they're now all out of a job. And even if what they did was legal, it still carried a stigma. Job prospects may be quite poor for a former privateer, and it certainly won't be as lucrative. So, quite predictably, after the wars ended, often times the privateers just kept doing what they were good at. So many pirates were simply privateers that had been cut loose by their respective governments.