Grooming inexperienced or demotivated freshers

I am a senior engineer with 4+ yoe I have few freshers under me and I have to guide/support them and set goals, and make sure they are billable to the project. Few of them do not have basic concepts clear for the role, not much active or motivated to learn or work. How do I guide them properly, assign tasks, so that they gradually start picking things on their own. I will definitely support them for difficult challenges. I also want to do this without being harsh or offensive. I need some ideas from the experienced managers.