For 0, the kanji is 零 and is pronounced れい (rei). Sometimes, it is also referred as マル (maru) which translates to “circle”. It is used like “oh” is used instead of “zero” in English when reading individual digits of a number. However, to pronounce the number 11 for example, you just have to add the Sino-Japanese reading of both the numbers, 10 and 1:  11 (十一) is じゅういち (Jyuu-ichi). Following the exact same rule: the pronunciation of 15 is じゅうご (Jyuu-go).