You are my best friend.
Welcome to the forum, Shiesha.
Swing gave you the correct answer with "Eres mi mejor amigo." (which is said to a male friend).
"Eres mi mejor amiga" (something you say to a female friend).
Eres mi mejor amigo - You are my best friend (masculine term)
Eres mi mejor amiga - You are my best friend (feminine term)
Quick lesson on Masculine v.s. Feminine:
All nouns in Spanish are either masculine or feminine.
Eg. El Libro = the book (masculine)
Eg. La casa = the house (feminine)
Generally speaking masculine nouns en in "o" and feminine nouns end in "a" There is no particular communicative advantage in having masculine and feminine nouns. I don't understand why all the latin languages evolved with such a seemingly pointless complication so you will have to learn a lot of extra infromation for no particular communicative advantage.
By the way some of them are not regular: (here are some very common ones) la mano = hand - la foto= photo - el día = day - el mapa = map - el programa = program - el sistema = system - el problema = problem -el clima = climate
Best Fried = mejor amigo