Under sink storage in the bathroom can be organized with many different types of systems. There are shelves and drawers which pull out for easy access that can be customized to fit any area. First you will want to measure your space to find the best option for you. Then take a look at what items you have and find something that can best hold your items. Drawers are best for small things and shelves can hold larger things.
Here is an example of an unorganized space.
Here is the same space using an expandable unit customized to fit your area and around your plumbing. This system has adjustable shelves and containers that can be used to hold smaller items.