I would like to turn a car which is in fbx turn into smoke. How can I turn the fbx into an obj?

It's simple.
If you open FBX file inside houdini follow these steps:
Press tab in object level and type geometry then dive inside.
Create a object merge and file cache nodes connect object merge to file cache.
Then give path of your geometry node inside object merge at object1 parameter.
Now go to file cache and with Geometry File parameter you can specify which directory you want to save.
Remember to put .obj suffix at the end of your file name.

Thank you.