I'm trying to determine the shared and exclusive taxa in my samples and to represent it by a Venn driagram. I read something about core-features, but I think that I'm not doing well, since I got something like this:
Let's go through it step by step!
First of all, since you want to get core taxa, you should collapse your feature table to desired taxonomy level (genera, as example).
Next step will be dividing your feature table on several separated tables according to the metadata column, that should be used in your Venn diagram (sampling site, treatment, source, etc).
Then you can run core-features command for each of obtained tables. Extract from visualization .tsv file at desired threshold (percentage of samples that should be considered as "core" (100%, 80% etc)), or just copy shared taxa from visualization in browser and use R, Python or online tools (Venny, InteractiVenn) to create a Venn diagram with taxa.