I had a look at a number of topic modelling tools. The first was Mallet, a tool frequently used for topic modelling. For instance, my collegue Emma Clarke, TCD and now NUIM, used Mallet to extract topics from the 19th century transactions of the Royal Irish Academy (on JSTOR). Her related blog entry is available here. For a detailed description on how to setup and use Mallet I recommand the blog post on the programming historian.
Another software that is quite popular for topic modelling in DH is the Topic Modelling Tool (TMT), and its use and examples are described by Miriam on her DH blog.
After searching a while the internet I found also a Python module, “gensim”, which claims to be for “topic modelling for humans”. It is not as easy to use as the above mentioned tools, but on is website there is a detailed tutorial, its developer Radim answered questions in a number of online forum, google groups etc, and also the API is very well documented. At a later stage I will use Mallet in order to compare the results that i get from gensim with another topic modelling tool.