I suggest you keep the focus of the lessons on getting students to speak and keep language work to a minimum. You can do this by giving the students a speaking activity and allow them to work in pairs or groups. It would be helpful to pre-teach some vocabulary that might be used during the activity, but try to keep this stage brief.
While they are speaking, you can go around and note examples of good work and errors that are made. Have a feedback stage where the students can summarise to the class what they discussed, then you can go over examples of good work as well as the errors you noted down. Let the students see the errors and give them an opportunity to correct them themselves. If you find that many students make the same mistake or that students make the same error every week, you could do some grammar work. However, the students have enrolled for a conversation class and so they will be expecting mostly conversation.
I suggest you take a topic that everybody can talk about for the first lesson, e.g. holidays. During the first lesson, find out how the students will be using English and in what situations. This will help you plan further lessons. As for the level, I suggest you contact the school or whoever is organising the classes. They will be able to give you an idea of the level; this will help you plan accordingly.
As for books, you can try: Discussions that Work by Penny Ur and Role Plays for Today by Jason Anderson.
You could take a look at this link to lesson plans for conversation classes on Tefl.net:
http://www.tefl.net/esl-lesson-plans/es ... ets-tp.htm