For some reason everyone knows that the Spanish for girls and guys are muchachos and muchachas. However, these are used in Latin America and should never to be used in Spain! Instead they say chicos y chicas:
María es una chica muy simpática – Maria is a very nice (likeable) girl
There’s also a colloquial and somewhat depreciative option chavales y chavalas which is used only with young people and only in Spain:
En mi clase tengo veinte chavales – There are twenty guys/dudes in my class
María es una chavala muy maja – Maria is a cool gal
When talking about others, people usually say tío/tía, menda (only in Spain) or (less frequently in Spain, more commonly in Argentina) pibe/piba:
Joaquín es un tío legal – Joaquin is a real dude
(Arg.) Marta es una piba guay – Marta is an awesome girl
The word tipo is used in both Spain and Latin America, it means bad or strange person:
¡Menudo tipo! – What a chap/bloke!
Also, it’s popular among young people to say colega, coleguilla:
Esta noche salgo con unos colegas – Tonight I am going out with some friends
English «mate», «colleague» or «co-worker» should be translated as colega or a rather neutral compañero:
Tengo unos compañeros de trabajo muy majos – My co-workers are very nice people
Talking to each other, both girls and guys often use macho, which is a type of folksy treatment, similar to patting one’s shoulder. Simply don’t translate it into English:
Oye, macho – Listen, (dude)
No sabes qué me ha pasado, macho – You’ll never guess what has happened to me
Instead of macho you can also hear colega, tío/tía in Spain, or compadre, cuate (México), pata (Perú), compay, hermano, chamaco in Latin America…
Among other Hispanic options for "guy", it´s worth mentioning güey, which used to be a swear word, but now it has various meanings, including «dude, mate», as well as mina, "girl", commonly used in Argentina and Uruguay.