A portfolio you can start by building whatever it is you would like to offer as a contracting service. Something that displays almost like a demo for what you could be hired to do while freelancing. More important than a portfolio though is references and quotes. Start doing work in your network for cheap and for a quote and reference at the end of the work. When you start building this you will not only build up a network of hopefully happy customers but also a portfolio of work you have been paid to do. The other option is to do 1099 work for someone or a company that has the contracts already for you to work on remotely.