Question: How do I get from London to StoneHenge & back, is it better to take a tour?
Answer: I think it depends on how confident you would feel using public transport. I suggest it is perfectly safe to use but you may find it easier to take a tour, especially one that collects you from your hotel and drops you back. (Tours are likely to be more expensive though.)
If you wish to use public transport, you can take a train from London Waterloo to Salisbury station. The train journey takes around 90 minutes, and trains run every hour. From Salisbury, you then need to get the number 3 bus, which leaves from near the station hourly and takes 40 minutes.