For starters all of that depends on the course itself and its not tough , you just need to have the right attitude required to pursue the discipline you choose.
Lets divide the courses into various types :
Professional
Research based
Professional courses would include courses like BCom,BBA,Law,MBA etc
Research Based courses would include BSc,BA etc.
One of the basic criteria is your marks in previous classes/courses,however more than the marks itself the attitude you have in the personal interview matters most.
How you present yourself in front of them carries a lot of weight when it comes to selection.