Planning an essay is not like planning a building, which has to be put up in a certain order if it isn't to fall down. You don't have to have done all your research in order to make a plan, and you will probably find that your plan alerts you to the need to do some more research in order to fill in gaps in your argument - and after that, you may need to redo your plan.
This isn't a waste of effort, because the process of planning helps you form a mental picture of the subject of the essay. It may assist the planning process to consider what the gist, or key issues, of the essay could be said to be, since this will help you define what approach you're taking.
Planning should be one of the early phases in the writing process, because it will help you work more efficiently. But planning is only a means to an end, and you need to be able to change the plan if the essay develops in an unexpected direction - you may need to replan the essay half-way through writing it if, for example, your argument is more complicated to express than you realized. A plan you can't actually use is always a bad plan, no matter how pretty it may look on paper.
You need to consider the essay's goals before you plan. To be helpful, the plan should have enough detail so that you know the direction your essay is going to take. In an essay on Wuthering Heights, for example, a plan which has as an item "the character of Heathcliff" is not going to be of much use - making this more specific (eg "the effect on Heathcliff of his orphaned childhood in Liverpool") will enable you to provide more signposting for your reader earlier on.