We have visited Megiddo several times by ourselves, and on several other occasions, we joined tours. I enjoyed our visits more during the times we joined tours. Since there are a lot of history and archeological layers, I suggest joining a tour. I saw two types of tours. The first is Christian-oriented (with many quotes from the Bible), and the second is archeology and history-oriented. I mention this since it is better to know it before you join. We have joined the free tours made by volunteers (mentioned above), which are of the second type. Also, see above the offer by Bein Harim.