There are tons of resources online - you don't need to be a businessman, just demonstrate examples of previous projects and how/where you created value for an employer/customer.

With the experience you listed in your post, I can't imagine you'll have too much difficulty getting clients.

I would do the following right away:

1. define your business plan - what services can you offer to which industries, and who do you want to target
2. create your brand - domain registration, email, simple website (lots of free website hosts available to get started)
3. create your online profiles - guru.com, elance, etc and fill in the website with lots of content on where you added value, gain results for a client and some name dropping to build reputation
4. create the marketing plan - who/how will you target, how to approach prospects, etc. I've seen a lot of resources that show easy ways to get some small client jobs to get started - ie approaching a restaurant owner and showing them how you can improve their marketing, social media, menu, etc etc - find problems with businesses in your local area and approach them with real solutions at a price that's hard to pass up. That builds the portfolio and gives you a base to build up with larger projects and customers.

I think every small shop PR/marketing consultant got started in a similar way and shared the same fears you might have now - many successful businesses were started out of fear, urgency and necessity. Just spend a few hours researching online and reading some books at Barnes/Noble, etc.