Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
I would use a CdS cell (photocell). This would be easier than an IR LED which would be your second choice. They should have a bunch of schematics online. So if you google "Line Follower Robot" or something, you should come up with a number of sources. With the results you get, make sure you check if it uses a CdS cell in the schematic.
CDS photo cells match human visual response better than most other types of sensors if I'm not mistaken. So white and black to you would be white and black to a CDS photo cell, where an IR sensor might be very different dependinig on the IR reflectivity of the 'white' surface.
Anyone that's ever played around with a black light knows that something that is white to the human eye can have dramatically different UV fluorescent abilities, and the same thing goes with IR light reflectivity but there's not a very practical demonstration to determine what an object will look like to a particular sensors without trying a variety of sensor types and determining which one is the best. Often changing the type of paper you're using, or the type of ink that's making the black line alone would be better than messing around with the sensor.