How can i setup lighting in Eevee? Im doing texture painting in Eevee, and I use lookdev mode. But i dont have a world shader and whenever i add any kind of light it doesn't make a change to my scene. What do i need to add / change in my world shader to be able to modify my lighting?
You need to enable your world shader in the Viewport Shading tab.  

Check Scene World to use your environment texture.
Check Scene Lights if you want your lights to show in LookDev mode.