These are suggested steps for what to do in the earliest stage of your project:
Starting off with a focused question will make your search for evidence easier because you'll already have an idea of your search terms. One way to form your question is to use the PICO model to define parts of your question.
After defining your systematic review question, it's a good idea to make sure there is a need for a new review on your topic. Doing a quick search of the literature helps to ensure that a similar review is not already in progress elsewhere, or has recently been published.
Some resources to check for similar reviews include: