Aim for your aim.
Screenwriting is a complex process and has many aspects. If you are outlining or researching, you certainly don't need to write in a specific format. But if you attempt the script itself, there really is no reason not to write in the standard screenplay format. On the contrary, forcing yourself to write within the limits of that format will help you think of your story in the right way.
For example, separating dialog from action forces you to write only dialog and action – and keep description (which is usually not part of a screenwriter's job) to a bare minimum. If you wrote freeform, you might be tempted to write what comes to your mind but is not part of a movie screenplay.
That you are not yet completely familiar with the screenplay format and find keeping to it difficult does not matter. Your first screenplay need not be perfect. Dare to make mistakes. If you avoid making them by writing freeform, you avoid learning.