Learn Italian: Le luci sono qui. - The lights are here.

luci (lights) is the plural of the noun luce (light). In Italian, nouns that end in -e in the singular always change to -i in the plural, whether they are masculine or feminine. Because luce is a feminine word, its plural takes the feminine plural article le.

Is there a rule to know if an -e noun like luce is masculine or feminine?

Unfortunately, no. You just have to memorize the gender of words ending in -e when you learn them. Looking at the article is the best trick: seeing la luce (the light) or le luci (the lights) immediately tells you it is a feminine word.

I thought sono meant "I am". Why is it used here for "are"?

The verb essere (to be) uses the exact same word, sono, for both "I am" (io sono) and "they are" (loro sono). Since le luci is a plural subject (they), we use sono to mean "are" in this sentence.

Could I say Le luci sono qua instead of qui?

Yes! Both qui and qua mean "here" in Italian. They mean exactly the same thing and are used interchangeably in everyday conversation, so either one is perfectly correct.