Once you learn kanji for Japanese numbers 1 through 10, you can describe 1-99 in kanji!
11 is “十一” (Think it as 10+1); 12 is ‘十二,’ 13 is ‘十三’…
20 is “二十” (Think it as 2x10); 21 is ‘二十一’ (2x10+1); 22 is ‘二十二’ … and so on.
After 100, you need another kanji 百, which will appear next:)