Hi @Lidia_Ryabova, there is a range of tutorials here: https://training.galaxyproject.org/training-material/topics/transcriptomics/. Most of them have workflows attached which you can use.
I am not really that familiar with RNA-seq, but maybe this one is a good start? https://training.galaxyproject.org/training-material/topics/transcriptomics/tutorials/de-novo/tutorial.html