Most Linux distributions have OCaml in their repositories; it is usually named ocaml. It's also in the ports of FreeBSD, OpenBSD and NetBSD. You can also download the source from here. See the README and INSTALL files for instructions on how to build OCaml.
The most recent Windows binaries for OCaml can be installed from here.
After successful installation, open a terminal and execute
ocaml. You are now in the Toplevel, an environment for interactively evaluation of Ocaml expressions and definitions, such as the following one.
3 + 4 ;; - : int = 7
;; ends an expression.