Open a new Part and make your changes. Save as > Part Template. Name it and Save. The default location will probably be in the SolidWorks installation folder. I'd suggest saving it to a different location so you won't lose it when you update to a new version of SolidWorks.
Point to this location at Tools > Options > System Options > File Locations > Document Templates.
When you start a new Part, the new template should be there to select. If it isn't, and you're getting the default options shown below, click on the Advanced button.
Start a new part, set the options and then overwrite your part template. Now after that all new part files should have those options set.