Cura is probably the most popular slicer program in use. It will create g-code files and can even be used to control your 3D printer directly.
While maintained by Ultimaker (a printer manufacturer), there are configuration files for just about any 3D printer out there. It’s free to download and use and is the recommended slicer and control program for many printers.
FYI, there are many different versions of Cura available. Start with the one that came with your printer. Once you have things working, you can try upgrading to the latest versions.