I am going to simulate borophene sheet using lammps. For that I need tersoff coefficient. I need create file for boron.tersoff B B how can I create it?
The format of the potential file for Tersoff is discussed in detail in the documentation for the tersoff pair style.
However, to write that file you need the parameters. Those you either need to look up in the published literature or learn how to parameterize yourself (not exactly a task suitable for beginners, though).
Please be aware that the Boron parameters of a BN potential file are not likely suitable. Classical potentials are not per-element like basis sets or pseudopotentials in quantum calculations, but rather specific to compounds and simulation conditions.