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.

normal	with dakuten ( ﾞ )	with handakuten ( ﾟ )
カ = ka	ガ = ga
サ = sa	ザ = za
タ = ta	ダ = da
ハ = ha	バ = ba	パ = pa

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.

normal	with chōonpu ( ー )	is the same as
プ = pu	プー = puu	プウ = puu