There are many ways to say that, but the easiest one would be "私の日本語はダメです(My Japanese is bad)", but if you really suck at it, then say "私の日本語は全然ダメです(not good at all)". Don't worry about the Katakanas, we use those to stress the meanings of the words.
For formal situations, say "私は日本語を勉強していますけど、文法(ぶんぽう)はとてもむずかしいです。".
BTW, まちがう(まちがえる) means to make mistakes, but does not mean bad. へた(下手)、ダメ or 全然ダメ、良くない、ひどい(酷い) woulbe be better.
Hello, mate.
My name is Oval.
I'm a Japanese that is studying British English now. So I need friend who can speak English. I'm looking for language exchange partner.I wanna know slang more and more!!
I may be able to teach Japanese.
My English isn't good enough yet... So if I am impolite, I'm sorry.
If someone interested, Please send me a message!
一緒に勉強しませんか?
I do not think you need to use the word "grammar".
If you make a lot of mistakes on grammar, that means your Japanese is not so good.
So, if I were you, I would just say...