I usually thank them for the interesting offer.
Since they are the ones coming to you so I would first ask them about details of the project, maybe past projects you can look into so you know they did something before, if you are interested in that and a inquiry about their budget and their goals for the project (if they seem realistic). But if they are going to pay you anyway (unless it is in %), that may not be your biggest concern.
Basically let them know you are interested, ask any details you are interested in, which opens the communication. If it is a company or someone bigger than you coming to you, they may very well be prepared with what they want from you and what the next step will be, so they will lead the conversation. They may have a contract prepared etc. So I don't think you need to think hard on how to proceed.
They may also ask for a meeting to get to know you and have the opportunity to explain everything and maybe ask for your rates there. You can also negotiate according to their budget (I did two nice commissions where the price I asked was doubled and I asked more than I ever got in the first place, so don't sell yourself low by telling them prices upfront, unless you have a reason to believe they won't be able to offer more or you want to be fair and ask the same from anyone).
Anyway, first they probably need to know if you are interested and if you have time.