A mercenary was just someone who fought for a private captain rather than out of duty to their local lord. In this sense, one could become a mercenary in pretty much the same way as one could fight in your lord’s wars. You just volunteered. The need for troops was constant, and there were never enough volunteers so there was no question of getting rejected. If you wanted to earn top dollar, you brought your own equipment, otherwise it may be supplied for you and taken out of your wages. If you had any experience you earned a little more, but even the greenest recruit was useful to stiffen the ranks behind the veterans in the front line. As long as you could hold a pike and walk without falling over too much then you were in. In terms of social status, you’d be a peasant of some kind. If you had any nobility in you then you’d be unlikely to join as just one of the lads, unless you wanted to hide your birth for some reason. In terms of background, it could be pretty varied. However the majority of people would be fighting because they had no other choice. It was a hard life, pay was barely enough to live on, and death from disease or battle was ever-present. If you could do anything else you probably would. So most soldiers were the scrapings of society; beggars, vagabonds or criminals.

There's more historical sources for mercenaries after 1300, where they seemed to become more common.

How did one become one? Often, a war would end or a intermittent warfare (Hundred Years War) leaving many soldiers unemployed. Many fought under the banner of their actual local lord and crown out of duty for their first war. So they packed up and hired themselves out to a foreign conflict rather than attempt to settle down peacefully in their poor former life. It was a tough life but had the potential for spoils and status. Unemployed soldiers were often encouraged to leave the lands because they had a habit of turning on their former employers/enemies and pillaging to survive; such as during The Hundred Years War.

Were they experienced? Yes, they often did have battlefield experience prior to becoming a mercenary, due to fighting in their native conflicts. War breeds warriors, especially in the martial less 'sedentary' cultures to the East (Alans, Turks, Pechenegs). A mercenary that has no worth in a battle is worth nothing. That isn't to say a company wouldn't have raw recruits enlisting after it was formed, though. Sometimes mercenaries were specific in their ability, from skirmishing to a "broken lance" heavy horse; and their region often dictated their fighting style and what made them useful. The Balkans and Aragon produced fine irregular light cavalry and infantry. The Italian states had men familiar with the crossbow and pike formations. German states and France had many companies that were very similar to a small medieval army, mixed with heavy cavalry, infantry, and archers. The list goes on.

Social status. Almost all except those who had land to enrich themselves with to begin with. People who did not have an established place in society, or any wealth to bind them to it. Second sons of nobility, landless knights, escaped peasants, outlaws, vagrants, and men who had lost everything from war. The lower classes often came from the fringes and outskirts of medieval life, from harsher lives and wild/war torn/border provinces (Marches). Many Almogavars came from Southern Aragon battling the Moors. The constant fighting and tribal nature of the Turks, Scots, and Irish had some enlist as mercenaries. The upper classes did so because they were unlikely to inherit anything unless older brothers died.

The famous English mercenary John Hawkwood started his life fighting for the English crown in The Hundred Years War. When the fighting simmered down, he became part of a "free company" and sold his services to the highest bidders. The condottiere are a very interesting set of mercenary history that this post does not give enough justice.

Here's some interesting wiki articles to start you off for further reading:

https://en.wikipedia.org/wiki/Catalan_Company

https://en.wikipedia.org/wiki/Roger_de_Flor

https://en.wikipedia.org/wiki/John_Hawkwood

https://en.wikipedia.org/wiki/Free_company

https://en.wikipedia.org/wiki/Great_Company_(German)

Further reading/sources

The Catalan Expedition to the East

Medieval Mercenaries: The Business of War (This is a cheap and good book)

The Hundred Years War Volumes

Medieval Mercenaries, The Great Companies (Very authoritative, hard to find)