another important note on kanji
before kanji were introduced, Japanese was a spoken language only, so there are some words, that are not written with kanji at all, or that won't correspond to the possible ways to read the separate kanji
an example of this is "today"
今日
which uses the character for "now" (今), that can be read as
コン, キン, or いま
and the character for "day" (日)
which can be read as ニチ, ジツ,ひ, or か
however 今日 is read as きょう

十一
is
じゅういち

二十
is
にじゅう

三十一
is
さんじゅういち

with Valentine's day coming soon, you might be interested in this video, by Japanesepod101.com
https://www.youtube.com/watch?v=S5KwK25yUNk
(note: we're not getting paid, we just think their content is good)

if you want a reference for how the kanji for numbers are written by hand, look at the order of strokes in these animated gifs

these are in order, from 1, to 10
for "one hundred" it is:

for "one thousand" it is: