Memorizing the Katakana/Dakuten

Dakuten ( ﾞ ) and Handakuten ( ﾟ ) are marks placed after certain katakana that modify the way the consonant is pronounced. These marks indicate that the consonant of the syllable should be voiced.

Exceptions
"Shi" (シ) turns into "Ji" (ジ)

"Chi" (チ) also turns into "Ji" (ヂ)

"Tsu" (ツ) turns into "Zu" (ヅ)

"U" (ウ) turns into "Vu" (ヴ)

Sokuon
Sokuon (ッ) is a symbol consisting of a small Tsu (compare with normal Tsu: ツッ). It is used to insert a slight pause.

Chōonpu
Chōonpu (ー) is a symbol used to indicate a long vowel sound. Long vowels can also be written by using the corresponding vowel katakana.