You will need a contractor. Here is how to hire one. Building permits required, electrical, plumbing and heating to be installed, not a simple DIY for most homeowners.
Yes to the contractor. Here are a few ideas to maximize space and the feeling of space. I try to do white walls or something very light. You may need to forgo a bathtub if space is limited. If you do a walk-in shower, try to do frameless doors with glass so the space will feel larger.
Permits and plumbing contractor.
Start with a drawing layout to scale.
This is where I am starting
You can make a full bathroom in a small space! Draw the layout on a paper, and just plan for a smaller vanity and shower