You don't know the correct pronunciation? If you had bothered to look it up, you would have found hi-RO-shi-ma, which is the "official" pronunciation, but digging deeper, you could find several variants, which are regional and/or local.

My best understanding is that heer-oh-shee-muh was two words at first, meaning "abundant island". Hi-roh-shuh-muh became one word, a noun, for the the city called "abundant island", which eventually became representative of the whole island, and also the city.

Hee-raw-shee-mah was the Japanese word for the city named "abundant island". Sort of like Cony Island became the name of a city, representing an earlier term for only the actual Cony Island, before the city became prominent.

Hirosh-ima is probably the best pronunciation that the average American tourist can manage. It's the best that I can do. The other pronunciations are usually tongue twisters for foreigners.