Hiragana are simplified kanji which are used as a syllabary. They are used to make sounds, but have no inherent meaning. Think of them like letters in English. Each letter represents a sound (but not a meaning), and letters (and hiragana) can be combined to make words. Hiragana are also used for imposing grammar on kanji. For example 食べる = Taberu. The kanji means "eat". Taberu is to eat. 食べたい = Tabetai. Tabetai means, "I want to eat" 食べた = Tabeta. I ate. 食べられる = Taberareru. I am able to eat. In each case, the kanji is the same, but the meaning is different. Katakana are used simply as a syllabary - to spell out words. In most cases, this is foreign words, such as "cake" (ケーキ). However, sometimes, katakana is used to emphasize a Japanese word - in a similar way to italics. If you are serious about learning Japanese, learn hiragana then katakana. After that, start learning kanji.

Katakana are also used for Onomatopoeias (the word that means a sound, like "boom") But usually they are for words imported from other languages to Japanese - for example パン

Katakana is used for load words of foreign origin