Hello, am new to lammps and I would like to know how to create an input script for a crystalline material in order to simulate and generate an XRD diagram. I would really appreciate if I can get a sample input script for example for silicon so that I can follow and apply to my system. Thank you.
Before asking this kind of question, please always first consult the LAMMPS documentation (if you do it in a smart way, you don’t have to read all 2500 pages of it 
).
I suggest to start here: 6.2. Package details — LAMMPS documentation
This lists possible commands and the path to a bundled example input.
You should first read the linked individual documentation pages, if you have questions about specific details, you need to look those up, too. Then you can study the example input (don’t forget to read the README file!) line-by-line and look up their commands and study their documentation, so that you understand what each is doing. And - again - if you have difficulties understanding details, look those up as well.
If you are very new to LAMMPS, you probably should also read through the chapters 1, 4, and 5 and perhaps practice with some simpler example input files first (see the README file in the examples folder).