Where are you located? The best advice for newbies is to figure out what brands/classes/engines are supported in your area and go from there. There’s no point in buying something you can’t get parts or setup help with easily, and there’s no point in buying an engine that doesn’t even compete at your local track.
As suggested above, visiting your local track on a race weekend is a great first step. If you give us your location I’m sure someone who is nearby can suggest what classes and tracks are around you.