I need expert opinion on planning phase and initial required documentation to develop a small to mid scale system

I have been hired to a brand new IT department with very dysfunctioning management and very inexperienced teamleader/ project manager. My responsibilities for department is as a entry level developer. my team (or just me) have given the task to develop a HR related management system and absolutely mess of a project planning phase. No proper use requirement gathering , system design and etc. this project been dragging along time with no progress what so ever. So im doing my own research to complete necessary steps to complete the initial planning and system design process so me and my fellow developers can start doing our own work. I need a experts guidance and advice on the dumb down version of what essential documentation and diagrams required for me to get to the part where i can code without any complication with me and my other developers.