Standard Softwares for Interior Designers:
AutoCad by AutoDesk. The full AutoCad application consist of both 2D and 3D production. However, in most projects, only 2D production is sufficient and required. Therefore, you may wish to use the light version of CAD which is AutoCad LT (light). AutoCad LT cost much less, easy to learn and require less computer memory and power.
Most of the time, 3D production in interior design is for the purpose of design and presentation and is not required for the production of final drawings. An easier program to use is Sketchup which is free and easy to learn. With Sketchup, color renderings can also be done as well. If you like to perform 3D renderings as well, it is a good idea to also learn the Adobe softwares such as Photoshop.
Both softwares are not hard to learn. The basics of Sketchup can be learned in a few hours. The basics of CAD can be learned in a few days. There are countless of free tutorials online to learn from.
Alternative Softwares for Interior Designers:
Instead of using AutoCad, LibreCAD and NanoCAD can be an alternatives. LibreCAD is free and is open source while NanoCAD has a free version and a paid version.
Instead of Sketchup, there is the Blender, Unreal Engine 4e and Unity.
3D Modeling and BIM:
Instead of ArchiCAD or Revit, there is the alternative free FreeCAD. Some learning curve and studying will be required for this powerful software.
Instead of Photoshop, GIMP can be used as alternative. This software is extremely similar to Photoshop.
Publishing and Typesetting:
Instead of Indesign from Adobe, Scribus can be used as alternative. It is open source and simple to use.
Instead of Corel Draw or Illustrator, there is InkScape. This software is a vector graphics editor of a professional quality. It is free and open code.
Documents, Spreadsheets and Slides:
Instead of using the standard Office softwares, LibreOffice can be an alternative. This software is very similar to the standard Office softwares and has all the features.
Instead of using MS Project, you can look at ProjectLibre.
Instead of the Adobe Premiere, Final Cut or Sony Vegus, therr is LightWorks can be used as alternatives.
Again, for most of the time, especially if you are self-employed, using just 2D drafting would be sufficient with some 3D modeling when needed.