Utilize all free platforms to grow your business. Sites such as Thumbtack which allow smaller players to get recognized. Hope this helps
For any creative small business (I'm also a small business owner for a creative business outside of my full time job!) you want to make sure you have a business plan (there are some great templates online). Use a business plan to identify short and long term goals and strategies for your business, and remember this is an evolving document.
As you create this business plan, make sure you include your social media presence. Interior design is very visual, and having a portfolio on your website, social media, etc. will allow people to save and see your work and hopefully come to you as a customer (it's a great way to get exposure). If you don't have a lot of expirirence outside of your education, make sure you build a visual portfolio, look book, mood board. Having visuals will help not only you, but your future customers understand your vision.
But your first step should be identifying your business goals and your plan, and developing a personal portfolio. Once you have that, you can go into registering your businesses, and building the social media presence to to start pushing your business forward.