Ask how much the retainer is and if you can pay with credit card.
Ask what the hourly rate is and if any of the work can be done by a paralegal in the office
Ask about the D process in your state -- how long is the required seperation, how long does it take a non-contested D to work thru the court system, etc
Ask if you can file for fault in your state and if there are any benefits in filing for fault
Ask how much you could expect to pay in total for a non-contested D that does not have any issues or delays
If you have kids then ask about CS
If you and your stbx make vastly different salaries then ask about SS
Ask why you should hire him/her instead of another L
Also, be ready to talk in general about all assets and debts. Have a basic idea about how many credit cards you have and the general balance, any car loans, any student loans, any other loans, how much money you have in investments, in 401K, in IRA's, in savings, any pensions you may have, general terms of your mortgage or rental agreement, etc.
Also be prepared to talk about what you want in the settlement and what is most important to you (example: if you want to keep the house or if you want primary custody of the kids or you to keep your own 401K and/or pension, or whatever is most important to you)