You can use the "region" command. E.g. for a geometry of unit dimensions arranged along the x-axis, you can write something like this:

region 1 block 0 1 0 1 0 1

region 2 cylinder x 0 0 1 1 2

region 3 block 2 3 0 1 0 1

To know where the numbers came from, you can refer to the LAMMPS user manual (link below).

Remember that before you can use the region command, you might need to specify a number of other parameters, such as units, atom types, and the boundary style of your global simulation box.

Hope that helps.

http://lammps.sandia.gov/doc/region.html

Good @Deepra