Not to fear. There are plenty of ways for you to figure out an appropriate number. You need to be prepared for the question at all stages of your discussions with the potential employer. You could be asked for your salary expectation at any point in the process, even in a preliminary phone interview.
How to arrive at that magic number? It is time to do some research. First, are you looking at an in house position, a law firm position, a government or non-profit position? What is the size of the entity (law firm, company)? What part of the country are you looking in? What is the industry sector (Insurance, IP, Corporate, etc.)? All of these variables will help you arrive at a number.
Typically, in-house positions, government and non-profit sector attorney positions pay less than large and mid-sized law firms. Federal government salaries are often determined on a pay grade basis, and are publicly available. State and local government salaries are also often publicly available, although they can be harder to find. In-house positions for larger corporations tend to pay less than large firms, and more than government positions. For smaller companies, in-house salaries are all over the map.