One (1) is 一 yī. Two (2) is 二 èr. Three (3) is 三 sān. Four (4) is 四 sì. Five (5) is 五 wŭ.
Six (6) is 六 liù. Seven (7) is 七 qī. Eight (8) is 八 bā. Nine (9) is 九 jiŭ. Ten (10) is 十 shí.
For example, 十四 shí sì is fourteen (14). Practice putting other combinations together.
For example, 二十五 èr shí wŭ is twenty-five (25). As you did for the numbers 11 through 19, practice putting different combinations together here.
A good way to practice as well as test your memorization of the numbers from 1 to 10 is to create flashcards with random Arabic numbers from 11 to 99. When you choose a card, write that number in Chinese.