Does anybody know how can make a program that calculates integrals?

I have an ingment at school(12gr.)to make a program (it doesn`t matter what programming language i use) that can calculate integrals. So my question is about the algorithm that i have to put into that program so it can calculate all types of indefinite integrals, only indefinite. Please help me with this, we`ve just learned about them 2 weeks ago and i don`t really understand...... It is very important for me because we get two grades on this project.. Thank you for you time to answer