In modern Japanese, as commonly known,「は」in 「はひふへほ」is pronounced as ‘ha’. But in a sentence that is 「〜は〜です 」, with「は」as a particle, 「は」is pronounced as ‘wa’. Why?
This is because after the 10th Century, Japanese started to pronounce 「は」as ‘wa’. So「かはいい」(cute) was pronounced as ‘kawaii’ and「こはい 」(frightening) was pronounced as ‘kowai'.
But the pronunciation of the first「は」and the「は」following the first「は」in words did not change. So pronunciation of 「はは」was ‘haha’. In 1946, the Ministry of Education changed the rule as it is confusing in modern Japanese.
Now we write ‘kawaii’ as 「かわいい」 instead of 「かはいい」。And pronunciation of 「わたしは日本人です」is ‘watashi wa nihon jin desu'.