When you are new, you do not have a long list of past clients to show as proof that you know what you are doing. I think the best way to get around this is to create a few sample projects that look exactly like the real work you want to do. If a client sees that you can solve their specific problem, they will care less about how many years you have been working. Apart from having a portfolio, what other ways can a beginner build trust with a new client?