"seminar.sty" is a LATEX document style for typesetting slides, and more.
You should know how to use LATEX, as described in Leslie Lamport's LATEX:
User's Guide and Reference Manual.
Let's get started:
If you are installing "seminar.sty", read the the accompanying read-me
file, and put the input files where your TEX looks for inputs.
Typeset the sample file, "semsamp1.tex", to see that everything is
working. You can use LATEX or AMS-LATEX.