I would call a few insurance companies and get their recommendations. One reason is because part of what they (the underwriters) may want to know is what your revenue will be, corporate structure, etc. - Some may advise that it can be covered under your general liability. I'm sorry - I know this is vague, but insurance companies have so many nuances that they need all of your particulars to help you make an informed decision. Naturally, even with your first client you may have some risk...but an agent should be able to help you weigh that risk.
For instance - if your first client was having you just get his or her files organized; little risk to them that you are going to cost them money by filing something wrong. However, if you are asked to book international travel and you send them to Japan instead of England...then that probably cost them a client as well as time & expense to get to their final destination.
I'm sure that helped about as much as grabbing a twig when your caught in quicksand...