Read the lesson plan for the week's topic and experiment. This information can be used as a guide to develop a lecture presentation and notes. Make sure to cover the topic succinctly with pictures and calculation problems so students will have a general idea of what they need to know.
Focus on the experiment next. In most university labs, the materials are already laid out for the class, but always make sure there are enough petri dishes, microscopes, swabs, etc. for the amount of students in the class. Make sure there are extra supplies for the experiment as well as for items students are responsible for, such as lab apron, gloves and goggles; there will be a few students who either forget or haven't bought the supplies yet.
Make sure the computer and the projector you will be using to display your presentation are working. Just because the computer is on does not mean it is working properly and will load the presentation correctly. Always load the presentation and check to make sure the pictures and animations show up.
Always do the petri dish or broth culture the students are expected to do in the experiment if they are expected to use the growth off their cultures in the next class. This ensures if some of the students' cultures don't grow or are contaminated, they can pick bacterial colonies off the teacher plates.