Kana chart

The two marks. ◌゙ (dakuten, two little ticks) turns a hard sound into its voiced pair, so the breath keeps going: か→が ka→ga, さ→ざ sa→za, た→だ ta→da and は→ば ha→ba.

◌゚ (handakuten, a small circle) is used on the H column only, turning it into a popped P sound: は→ぱ ha→pa. The kana shape never changes, so once you know か you can read が too.