Romanji to Japanese transliterator

[simonefrancia]

Hi,
I am new with Japanese Language and I am discovering a lot of interesting thing about this language.
I am trying to transliterate romanji to Japanese (kanji, hiragana, katakana) and the task seems to be very good.
But, when I propose transliterated sentences to my Japanese friends, they tell me that they are not so good and they don't understand the meaning of the sentence.
Could you give me some explanation about these problem and also some advice to get better performance?

Thank you