I am going to categorize texts into predefined topics. It looks like Active Learning approach is suits for me. Are there any good tutorials or advices?
While I'm not aware of any tutorials, I can provide a bit of advice from my past experience.
Research regarding active learning has tackled many different aspects of this domain, but not all of them might be relevant to you.
Start with rather simple approaches, like alternating random sampling and uncertainty sampling in an appropriate interval. It is a quite simple approach that can reach a good compromise between exploitation of already acquired labels and exploration of the unknown parts of the data space.
If such an approach turns out as requiring more class label acquisitions as you are willing to invest, you can look for more elaborate approaches. You can have a look at this survey article that explains the basiscs of active learning and describes the different directions the research takes in this domain.