First of all, there are three versions of Gammess: UK, US and Firefly. I will assume that you are using Gammess US.
Take a look at the examples at the manual.
You have to put the coordinates of the atom. It is very simple in your case because you have one atom, that you can locate at $(0,0,0)$.
You also have to set your basis.
Usually I explicitly write it in the input after taking the basis from Basis Set Exchange but since you have such a simple basis you can use the ones bundled with Gammess US. Furthermore, the basis is not a complete description of the model you are going to use. That should be explained in the book so I suggest you to read it carefully. You make an ansatz about your wavefunction, may it be monodeterminantal, expansion of deteriminants or other you will be using a model like Hartree-Fock, Density Functional Theory, Configuration Interaction, Coupled Cluster, explicit methods, ...
I don´t know which you are going to use but probably it is Hartree-Fock, since it is only to get a grasp. Caution: Li, Na and K all have odd number of electrons so they are not closed shell. Instead their ground state is a doublet in all of them. Knowing this and that you are only interested in the density and not in the spin-density you can do restricted Hartree-Fock (RHF) while notifying that it has one unpaired electron.
Isolated atoms have spherical symmetry but the closest symmetry you can get in Gamess is octahedrical ($O_h$) so use it.
In addition you have to let the program know that you want to have the information of the wavefunction printed at the end of the program execution. Then you have to set the
AIMPAC=true variable. Then you can use, lets say, Multiwfn to plot the density isosurface.
Also you will find the following tutorial useful.
The visualization program Avogadro has an input generator but you still have to know what you are doing, it is only a tool to help you to create the file