Dakuon  だくおん
There are 20 Dakuon letters in Hiragana.  Dakuon is a Hiragana letter with  2 dots " 
が ぎ ぐ げ ご  ga  gi  gu  ge  go
ざ じ ず ぜ ぞ     za  zi(ji)  zu  ze  zo
だ ぢ づ で ど    da  di  du  de  do
ば び ぶ べ ぼ   ba   bi  bu  be  bo
* が has 3 dots.  However, Katakana Dakuon ガ has 2 dots.  
* じand ぢare the same sounds.  ずand づare the same sounds. Even though the roman letters are different, Japanese people pronounce them same. 
Handakuon はんだくおん
There are 5 Handakuon letters in Hiragana.  Handakuon is a Hiragana letter with a circle 。
ぱ ぴ ぷ ぺ ぽ  pa  pi  pu  pe  po
Youon ようおん 
There are 36 Youon letters in Hiragana.  Youon is a Hiragana letter with small ゃ ゅ ょ
Read き and や slowly.  Then, read  き and や very quickly.  That is the sound of きゃ
 
きゃ きゅ きょ kya  kyu  kyo
しゃ しゅ しょsya  syu  syo (sha  shu  sho)
ちゃ ちゅ ちょtyatyutyo (cha  chu  cho)
にゃ にゅ にょnyanyunyo
ひゃ ひゅ ひょhyahyuhyo
みゃ みゅ みょmyamyumyo
りゃ りゅ りょryaryuryo
ぎゃ ぎゅ ぎょgya  gyu  gyo
じゃ じゅ じょzya  zyu  zyo (ja  ju  jo)
ぢゃ ぢゅ ぢょ dya  dyu  dyo 
びゃ びゅ びょbya  byu  byo
ぴゃ ぴゅ ぴょpya  pyu  pyo
 
*じゃじゅじょand ぢゃぢゅぢょare the same sounds.  Even though the roman letters are different, Japanese people pronounce them same. 
Hiragana Dakuon Handakuon Youon Chart 
                                                Hiragana Chart with words
                              Click  あいうえお(教科書体) 濁音 半濁音 拗音あいうえお表1
                                         Simple Hiragana Chart (Dakuon Handakuon Youon)
                  Click 
ひらがな濁音拗音一覧表(PDF) 
                    Click ひらがな表 濁音なぞり書き2
                                                                 Youon Worksheet
                         Dakuon Handakuon Youon Worksheets
       2 letters in each page  Total 31 pages
                    There are good for younger children and beginners