Japanese Onomatopoeia

Two kinds

Giongo (擬音語) imitate real sounds: わんわん (a dog barking), にゃー (a cat), ざあざあ (heavy rain).

Gitaigo (擬態語) describe a state or manner that makes no sound: きらきら (sparkling), どきどき (heart pounding), にこにこ (smiling).

Everyday examples

ぺこぺこ: おなかがぺこぺこです。(Onaka ga pekopeko desu.) I'm starving.

ぺらぺら: にほんごがぺらぺらです。(Nihongo ga perapera desu.) Their Japanese is fluent.

ぎりぎり: ぎりぎりまにあいました。(Girigiri maniaimashita.) I only just made it in time.

ぐっすり: ぐっすりねました。(Gussuri nemashita.) I slept soundly.

しとしと: あめがしとしとふっています。(Ame ga shitoshito futte imasu.) A soft rain is falling.

How they fit in a sentence

With と, as an adverb: あめがざあざあ(と)ふっています, it's pouring.

With する, as a verb: どきどきする, to feel your heart racing.

With です or だ, as a description: おなかがぺこぺこです.

Hiragana or katakana

Both are used. Manga often writes sound effects in katakana (ドキドキ), and everyday writing often uses hiragana.

The meaning is the same in either script.

How to learn them

Learn each one with the words it usually goes with: おなかがぺこぺこ, にほんごがぺらぺら. On its own, a sound word is hard to use.

Start with the handful above. They come up in daily conversation, not only in manga.