What do I need to legally work abroad?

A work visa for that country

You will need an employment visa. Only QUALIFIED employers can apply for employment visas for their qualified foreign employees. So what are your qualifications? Which country lists those qualifications as being in short supply there & eligible for visa? Who are the employers there which are qualified & authorized by the govt to hire foreign workers & apply for any employment visa?

E.g. India is terribly short of doctors, medical specialists, yet it is nearly impossible for any Indian company/organization to hire any foreign workers, not even an experienced MD with a needed specialization.

Typically, for most countries now, expect to need a minimum of a master's degree + experience in a STEM field. And expect a lot of competition from around the world for any job which holds the possibility of an employment visa. Worldwide labor glut of a billion adults already with another billion on track to enter the world's workforces within the decade. There aren't enough jobs to go around, and lots of occupations are shedding job positions, categories like crazy as technology makes more occupations obsolete. Hardly any country (except US) is stupid enough to let anybody in, even temporarily to work, unless they have a severe need for workers with some particular specialist skill.

You need whatever the other country requires. It could be a visa, it could be a work permit, it could be a bribe. It is different for each country.

That varies by the country.

Unless of course your'e looking for sex work.

An advanced degree in a STEM and year of work experience and an offer of a job from a registered employer who is prepared to sponsor your work visa is the starting point

http://www.ef.com/blog/language/want-to-work-abroad-12-things-to-do/

your bags and papers

So many vaccinations for different diseases found outside the U.S.
Be ready to become a pin cushion.