In Sweden you can never go wrong with a simple hej as a greeting.
Whether you're talking to your friend, a child, an adult, your boss or a complete stranger – hej always works, and is the safest bet if you are not sure what level of formality to aim for.
But if you're confident enough to branch out into the great wide world of slang, you may want to consider going for a tjena. It is more familiar than a hej and is not appropriate for greeting a prospective employer in a job application, but as Swedish is not a particularly formal language, it works in nearly all other contexts.
Tjena is short for tjenare which can also be spelled tjänare.
It actually does come from the word tjänare (pronounced with a slightly longer äää), which means 'servant', and the greeting can be traced back to around 1775 in its longer version mjuka tjänare ('humble servant').
Nowadays, few Swedes reflect on this archaic meaning of what is considered an informal greeting phrase rather than a subservient way of showing civility and deference to a very important person.
Other, even more informal, variants include tjenixen, tjenamors, tjabba or tja.
Tjena kexet, står du här och smular?
Hey there biscuit, are you standing here crumbling? (a Swedish pick-up line based on the fact that the word kex can refer both to a biscuit and to an attractive man or woman)
Tjena Olle, det var inte i går!
Hi there Olle, long time no see! (literally: Hi there Olle, it wasn't yesterday [that I saw you])