Many of the phenomena ocurring in the early universe are characterized by the presence of large non-linearities in the field evolution, hindering the use of analytical techniques to obtain precise predictions. One example is cosmic strings. These are topological solutions of the field theory generated after phase transitions in some extensions of the Standard Model, such as in some axion models. Typically, they are analytically studied using the Nambu-Goto equations, in which the width of the strings is neglected. In this talk, I will explain how one can use classical field theory lattice techniques to study all kind of early-universe phenomena. I will later show some preliminary results on the gravitational wave emission from cosmic strings, and how they compare to the very simple Nambu-Goto predictions.