A lab report is more than just something you turn in to (hopefully) get a good grade. It's your opportunity to show that you understand what is going on in the experiment, which is really the most important part of doing it. In addition, I think it's actually very good practice for getting across your thoughts about the science you are doing in a manner that the reader can understand.
What you write in your laboratory notebook is an actual account of what you have done in a given experiment, like a very detailed diary. You should be able to come back to it at some point, read what you wrote before, and reproduce what you did before. So should anyone else reading your notebook, for that matter. That way, if you make some amazing discovery, like blue aspirin is better than white aspirin (btw: don't eat anything in, from, or created in lab to see if this is right), you will have a permanent record of it to remind you of your greatness. There are three basic parts to a lab report: pre-lab, in-lab, and post-lab. In this document, I've written some helpful tips that might help you through your lab-report woes. I won't include everything you have to do (you should look on VOH for the report guidelines), but just a few key ideas.