Hi! I'm actually studying Japanese in school and I might be able to help. Katakana is only used for English words or words that are not derived from the basic Japanese language. For example, your name would be written in Katakana. Certain characters can change but that's when you start entering kanji. For example "to go" is いきます。(ikimasu) but when you start getting into kanji it will look like 行きます。 (I hope that's what you're questioning about.) As for memorization, you really just kind of have to do practices everyday even though it might not seem like the most fun thing to do. The more that you kind of force yourself to practice it, the more you'll recognize it and learn it.