Well, an interaction is simply a multiplication of two (or more) variables. So, if your model is y=a+bx1+bx2+u, then the model with interaction would be y=a+b1x1+b2x2+b3x1x2+u.

Keep in mind that adding interactions (could) increases the multicollinearity. A good reference book, from my point of view is:

Aiken, L. S., & West, S. G. (1991). Multiple regression: Testing and interpreting interactions. Sage.