My bad Japanese :(

Konnichiwa!

watachi wa Elena desu, --> Watashi wa Elena desu.

demo nihongo ga hiragana/katakana/kanji to yaku dekinai....

--> Mada nihongo ga heta desu. (My Japanese isn't good yet.)

Hiragana to katakana ga wakarimasen. (I don't understand/know hiragana and katakana. = I can't read nor write hiragana and katakana.) 

*You don't have to worry about kanji, please learn hiragana first, then katakana, please. 

chotto muzukashi desu... --> Chotto muzukashii desu. 
Ganbarimasu!!! (Correct!) 

Good job! I hope this helps you. Good luck!