The trivial answer is you install the Git plugin. You could also install the GitHub plugin. Basics are covered in the details of the plugins pages.
There is also an additional guide on the Jenkins site - Jenkins with GitHub. Other sites also have examples. In the build step you would put the command to execute; root repo is in in the workspace root.
But what if you already pull down a repo and now want the one with the script or csv? It sounds like you have the script and csv in separate repos (probably a good thing). Basic freestyle projects don't allow multiple repos into a single workspace. For that you need a pipeline. Details in Jenkins, or of course, in Stackoverflow.
There are alternatives. You could have one job pull the tools (powershell) repo to one location and your other jobs refer to it from that. You could use curl or wget to get the one file from the repo. You could get more complicated and make the tools repo a sub-module to your base repo (not a fan of sub-modules; ymmv).
Pipeline is probably your best bet for multiple repos.