"Nosotros" and "Nosotras" are the first person plural, as the English word "we" but "Nosotros" is masculine and "Nosotras" is feminine. "Ellos" and "ellas" are the third person plural, as the English word "they" but "Ellos" is masculine and "ellas" is feminine.

muchas gracias :)

You can use "nosotras" If you're a woman for a group of women, and "nosotros" If you're a man, for a group of mens. You should say "nosotros" for talk about a group of men and women included you. For "ellos" and "ellas" the use is similar but with the third person.

you have to know the gender of noum you're going to use and choose "el" or "ella" if it's male or female... There is no specific rule so you have to use a dictionary till you know them... it's just practice...