みらいさん、私も日本語を勉強しています。When I learned Katakana, the following is how I pictured some of the mentioned characters:
シ：I thought about someone smiling. If you try to imagine the character as a smiling face, that might help. "SHI is smiling"
ツ：For this, I thought of it as a "tsu"nami. If you picture it, you can think of water falling or surging.
ノ：For this, I always think of a dog's nose.
It's not much, but hopefully that helps.