I’m modeling a house just to practice a little and when I finished making the bottom floor I discovered that I didn’t make groups or anything like that so I could separate the parts and furnish them. Can anyone help me with this? Is there any extension or something like that that will make this process easier or is the only thing left to do is group the parts manually?
I’m very new to sketchup 
Here’s the project if you want to take a look…
You need to start with the basics
To help achieve your goals, some time spent at the SketchUp Campus and at the SketchUp - YouTube channel will be very worthwhile. Both sites are from the SketchUp team. On the YouTube channel, pay attention to the Square One Series and Level UP series. They cover the basics for each tool.
There are extensions that help you to create walls or different elements without having to make groups like profile builder or Dibac that will do that automatically, but the best way to practice and getting used to the program is to model using native tools and develop the muscle memory of grouping or making components right after modeling something.