(Corrected sentences) I need to study more. 私はもっと勉強する必要がある。
私はもっと日本語を勉強したい。
I want to learn more Japanese.
それは私にとって難しい。
It is hard for me.
*I would say 日本語は私にとって難しい (Japanese is difficult for me.) or 日本語を勉強することは私にとって難しい (Studying Japanese is hard for me.), however it's only my opinion.
私はアニメを見てきた。
I have been watching anime.
If you still keep watching it, it's better to say 私は(以前から)ずっとアニメを見ている.
時には、それは便利です(or 役に立ちます)。
Sometimes, it is helpful.
*便利です is correct but you wrote other sentences in the informal form, so it's better to use the same form, 便利だ or 役に立つ。
Hi, great job! If you have any doubt in my suggestion, please let me know. I hope this helps you.