Saving bank account turnover

dear sir my query is....if I am showing my turnover is Rs 15,00,000 and paying tax on 15,00,000 under sec_44ad I.e. nil tax but my question is whether I have to opt service tax no. or go for current account because my s.b. threshold limit is Rs 10,00,000 whether will it create any problem in next year if my s.b. saving has transactions 1200000 cash from checque and 800000 from cash how much should I pay tax in next year .....suppose I am a furniture contractor and hire labour for work. plz help me