As you have started as an SEO consultant. I hope you may be having your own website, If not create a website for your business. Do SEO for your site and rank for the keywords for which people are searching for SEO services. It will also prove your credibility in ranking on the search engine. If you are not able to rank your own website then don’t expect to get clients.
In the portfolio, you can mention your own website results. There are many people who are looking for genuine results.