Explaining to novice users how to interact in immersive VR applications may be challenging. This is in particular due to the fact that the learners are isolated from the real world, and they are asked to manipulate hardware and software objects they are not used to. Consequently, the onboarding phase, which consists in teaching the user how to interact with the application is particularly crucial. In this paper, we aim at giving a better understanding of current VR onboarding methods, their benefits and challenges. We performed 21 VR tutorial ergonomic reviews and 15 interviews with VR experts with experience in VR onboarding. Building on the results, we propose a conceptual framework for VR onboarding and discuss important research directions to explore the design of future efficient onboarding solutions adapted to VR.