Almost every 3d modeling software should be capable of exporting .obj files
If you're using SketchUp, it looks like you need the Pro version to export to obj: https://help.sketchup.com/en/sketchup/exporting-obj-files
Alternatively you can check out Blender, it's an extremely powerful tool and it's available for free, but controls can be tricky and it may take some time to get used to it 
If I were to make a house in Sketchup and open it in Rising World, would I have to build the house just like I would if I were to build in the game? For example, in Sketchup, I would have to design and combine planks and beams, blocks, etc to make the house, so when I export to Rising World, the'll be no errors? Or will Rising World automatically convert the structures(like walls) to planks, beams, etc?