Basically you want to use both-inbound and outbound marketing.
Start small, if you already had your customers, for example from the freelance websites and you still keep in touch, you should ask for the feedback.
You can start with a landing page with clear description of the services you provide, and the reviews from your previous customers. You can make case studies for each of them to describe the complexity of the following tasks and how you solved and acheived your results.
The easiest way could be facebook ads, it is easy to make few targeted campaigns there and test which work best for your purposes.
Another way to claim your professionalism: creating your personal brand on social media and continuosly advice and create lead-magnet content, that will engage people to trust you as an expert.
And 3rd but not least: As you start your company, work on vision, mission and objectives, understand your ideal customer and the price that you are selling your services. And go for it: make a list f the companies that can be your relevant customers and reach their decision makers through Linkedin/their website and other forms of communication.

Will be glad to shortly give you the advice on the best leadgen tactics and discuss the materials that you already have starting your company