I am planning to do a "Suggested Topics" block behind topics on my web forum and now I am looking for advice on this. Which recommender engine or techniques do I need to use for this task?
I understand that this is a problem of clustering but I am not worked with this area before.
No, instead of clustering you should just use an actual recommender system.
Or you just use search, such as Apache Lucenes "more like this" function.