REGARDING STCG

I am a student and there is no other source of income, I have invested in stock from my pocket money. I earned 15000 from stock market in Last year. I am not paid short term capital gain. (My friend told me if your income less than 150000 you do not have to pay any tax. that income comes under basic tax exemption limit. I am warring what will happen if I do not pay (because of lack of knowledge) pls suggest me how I do??