Dakuten and handakuten are marks added on top of hiragana that alter pronunciation.
The dakuten mark is ( ゛). There are 20 dakuten variations. For example: か (ka) → が (ga).The handakuten mark is ( ゜). There are 5 variations. For example: は (ha) → ぱ (pa).
See this useful diagram for the general pattern:
K → G
S → Z
T → D
H → B
H → P
か (ka) → が (ga)
さ (sa) → ざ (za)
た (ta) → だ (da)
は (ha) → ば (ba)
は (ha) → ぱ (pa)
き (ki) → ぎ (gi)
し (shi) → じ (ji)
ち (chi) → ぢ (di)¹
ひ (hi) → び (bi)
ひ (hi) → ぴ (pi)
く (ku) → ぐ (gu)
す (su) → ず (zu)
つ (tsu) → づ (du)²
ふ (fu) → ぶ (bu)
ふ (fu) → ぷ (pu)
け (ke) → げ (ge)
せ (se) → ぜ (ze)
て (te) → で (de)
へ (he) → べ (be)
へ (he) → ぺ (pe)
こ (ko) → ご (go)
そ (so) → ぞ (zo)
と (to) → ど (do)
ほ (ho) → ぼ (bo)
ほ (ho) → ぽ (po)
(1) ぢ is written in romaji as di but is pronounced ji.
(2) づ is written in romaji as du but is pronounced zu.