If you have received an offer- congratulations! Maybe you are jubilant, maybe you are weighed down with reservations, maybe you are even thinking about turning it down.
When offered a position, the first thing you should do is express your gratefulness to the offer. Even if you are holding out for a better offer, even if the offer is not at all what you wanted, thank them graciously, and ask how much time you have to make a decision. You want to avoid mentioning that you are waiting for an offer from a different firm, or are unhappy about their offer. Simply say thank you, and tell them how much you appreciate their offer to you. Then hang up the phone, and take some time to digest what has been offered to you, and how that stacks up with what you are willing to accept.
Some law firms will give you their first and last offer up front. That is because some firms have lockstep salaries for associates and/or strict formulas for determining partner salaries. If you are lucky enough to be working with a recruiter, they should be able to tell you what kind of a firm you are dealing with.