A noun in Italian can be either feminine or masculine. Generally nouns ending in -a are feminine, while nouns ending in -o are masculine:
masculine = ragazzo (boy), libro (book)
feminine = ragazza (girl), borsa (bag)
There are some exceptions:
la mano, la radio, la foto, la moto are all feminine even though they end in -o
il problema, il guardaroba, il cinema are all masculine even though they end in -a
There are also nouns ending in -e, these can be either masculine or feminine, you just have to learn the gender while learning the word…
masculine = il cane (dog)
feminine = la stagione (season)
Nouns ending with a consonant or borrowed from another language are usually masculine:
il bar, lo sport, il computer…
Usually to make a masculine noun feminine, you only have to change the -o into an -a:
amico – amica
ragazzo-ragazza
bambino-bambina