I am travelling to Portugal from India to attend an academic conference where I will be presenting a paper. I am an undergraduate student and not employed (or receiving payment for attending this conference). Should I apply under Business category or Tourist Category?
If your sole purpose in making the journey to Portugal is to present a paper at an academic conference, then you would indicate 'Business'. To assure that your application is treated as a genuine business visitor, your proposed travel dates should not extend wildly beyond the overall conference dates. They will also be looking for something that connects your programme of study to the conference topics and you may wish to include something explicit as evidence (it's optional but may help).
You can include the conference details and your spot on the conference proceedings as supporting evidence along with all the other evidence needed to support your application.
There's some generic information on Schengen applications here: Schengen Visa Application: tips for preparing