In most mediations, you won't need a lawyer right there with you. This is because the parties are trying to work together to solve their problem -- not trying to convince a judge or arbitrator of their point of view -- and because mediation rules are few and straightforward. If your case involves substantial property or legal rights, however, you may want to consult with a lawyer before the mediation to discuss the legal consequences of possible settlement terms. You may also want to make a lawyer's approval a condition of any agreement you reach. For more information, see Nolo's article Mediation: Do You Still Need a Lawyer?
For a detailed guide to settling divorce, business and employment disputes through mediation, get Nolo's eBook Mediate, Don't Litigate, by Lisa Guerin and Peter Lovenheim (Nolo).