What part of "no" don't you understand?
The Japanese are famously reluctant to say the word "no", and in fact the language's closest equivalent, いいえ iie, is largely limited to denying compliments you have received. ("Your Japanese is excellent! "Iie, it is very bad!"). But there are numerous other ways of expressing "no", so here are a few to watch out for.
Ii desu. Kekkō desu.
"It's good/excellent." Used when you don't want more beer, don't want your bentō lunch microwaved, and generally are happy to keep things as they are. Accompany with teeth-sucking and handwaving to be sure to get your point across.
ちょっと難しいです･･･Chotto muzukashii desu...
Literally "it's a little difficult", but in practice "it's completely impossible." Often just abbreviated to sucking in air through teeth, saying "chotto" and looking pained. Take the hint.
"This is inexcusable but..." But no. Used by sales clerks and such to tell you that you cannot do or have something.
"It's no good." Used by equals and superiors to tell you that you cannot do or have something.
"It is different." What they really mean is "you're wrong". The casual form chigau and the Kansai contraction chau are also much used.
Leave me alone. ほっといて。 (hottoite.)
Don't touch me! さわらないで! (sawaranaide!)
I'll call the police. 警察をよびます。 (keisatsu o yobimasu)
Police! 警察! (keisatsu)
Stop! Thief! 待て! どろぼう! (mate! dorobō!)
I need your help. たすけてください。 (tasukete kudasai)
It's an emergency. 緊急です。 (kinkyū desu)
I'm lost. 迷子です。 (maigo desu)
I lost my bag. かばんをなくしました。 (kaban o nakushimashita)
I dropped my wallet. 財布をおとしました。 (saifu o otoshimashita)
I'm sick. 病気です。 (byōki desu)
I've been injured. けがしました。 (kega shimashita)
Please call a doctor. 医者を呼んでください。 (isha o yonde kudasai)
Can I use your phone? 電話を使っていいですか? (denwa o tsukatte iidesuka)