planning your project before you code

Planning and preparation is one of the main areas I need help with. Sometimes, I just don't know what the planning process looks like; and I'm not sure if the correct information is being researched. If I wanted to build a new app for the first time, and I wasn't sure how to build it correctly. What would the process of planning and preparation look like? for example, if I wanted to build an app that allows users to log-in, post comments, and view them on a blog. How would I go about researching a project like this before I start coding?