Stuart has good corrections. I'd mention two things: First, I'd say, "This dish smells good, but I'm full." ("Full up" sounds really weird.) Second, just wanted to mention that "petrol" is called "gas" or "gasoline" in the US. Just in case you come across it, they're the same thing! :)

Hi Hsing, pretty good sentences, just a few small errors with prepositions and spellings. This dish smells GOOD but I'm FULL up. I can't eat ANY MORE. ( Anyone is a person! ) The car doesn't have petrol. We have to fill IT up. ( Use the pronoun to avoid repetition, it sounds more natural ) I'm specialized in this field. But THAT field is totally another story. ( Don't need 'in' without the verb ) You are already behind schedule. You have to think how to catch up. ( Don't need the article with schedule, as it is a concept ) I have received your mail, but I haven't had time to catch up WITH the problem yet. ( 'Catch up' needs the preposition 'with' ) Please give me some time. I'll GET BACK TO YOU as soon as possible. ( 'Get back to you' = reply, 'Get you back' = get my revenge! ) Very good natural-sounding sentences though, well done!