Once you've hooked your reader with a good introduction, you'll start getting into the details about how you performed your study or experiment. This section should be written with enough detail that anyone would be able to follow your procedures and repeat your experiment. But make sure you don't include so much detail that it becomes overwhelming!
The Materials and Methods section is often the easiest part of a lab report to write because the procedure is either written in your lab manual, or you took notes on your procedure as you performed the study. Just make sure you write it in paragraph form with complete sentences, rather than just a list of your methods. As with the other parts of the paper, this section should usually be written in past tense with no personal pronouns (I or we).
It's very important that in the Materials and Methods section you write only what you did, not what results you got. Save those for the next section.