Every famous writer had to sell their first novel before they made it big. Everyone stars at the same place, and it is nothing to be ashamed of. If you lack publishing credentials, don't let it stop you from querying top agents in your genre. What matters is that you can write a convincing query to let those agents know your book is worth reading.
Agents are in the business to make money. It is as simple as that! If an agent has confidence that they can sell your novel, they will make you an offer of representation. Polish and edit your novel to the best of your ability before you query, so that you are putting your best foot forward.
Do your research before you start sending letters to prospective agents. If you write fantasy and query agents who don't represent that genre, you will get form rejection letters. Find agents who represent the books most similar to yours, and begin your hunt with them.
In many cases, it can actually be easier to sell a first novel than a second or third. If your earlier works had disappointing sales and did not earn out your advance, many publishers will be wary about giving you a second attempt. If an agent believes in your work, they may be willing to help you work around these obstacles. It could be as simple of a fix as choosing a pen name is in order to distance you from previous works.
With nonfiction, it may not be important to have publishing experience, but you should have a "platform" on which to build your book. Most publishing houses prefer authors who have an expertise or authority on the subject. Anyone can write a diet book, for example, but your odds are far better if you are a respected nutritionist or physician. Have a good reason why you are the right person to write your book before you create the proposal.
When you send your query letter, do not draw attention to the fact that you have no prior publishing credits. There is no need to specify that this is your first book - just omit the information entirely. If an agent does not see listings of previous credits, they will assume you are a new author. It's ok! Make sure your query letter is engaging and intriguing enough to convince the agent to ask for the first few chapters. Unless the agent's website or listings specify otherwise, include the first three to five pages of your novel as a writing sample.
However, you may want to consider checking up on your web presence. As the internet becomes more prevalent in the publishing industry, many agents now Google the names of potential clients to see what comes up. They just might stumble across your Helium articles, so make sure that they showcase your best work!
Like all authors, you will face rejection. Do not be discouraged! It is not uncommon for new writers to query dozens of agents before finding their perfect match. Work hard and listen to critiques, and your writing will find the right home.