Please read the DESeq2 manual, I see do point repeating here what is already in there. Feel free with specific questions once you went through it in detail.

This is a common step to be at. I'd recommending stepping back from the data and defining what you want answered. eg. Do you want to compare pairs of groups, maybe each group versus a control group? Do you want to answer whether the gene varies at all across all 6 groups? Do you want to compare groups AB vs CD and EF, etc.

Usually the biology/experimental design will dictate some obvious, meaningful group comparisons.

Settling on a question(s) will let you define the comparisons which will determine what you need to do in DESeq2. https://rdrr.io/bioc/DESeq2/f/vignettes/DESeq2.Rmd section "Differential expression analysis" has some basics on this, and looking up other DESeq2 vignettes with "comparison" and "contrast" examples will help. If it's still unclear you could try asking a more focused question. Good luck!

You should run DESeq and determine the appropriate contrast for the groups you are comparing. Refer to my response to your other question for details on how to identify the correct contrast: DESeq2 analysis