how to add tags and segments to customers imported via .csv or manual

I am trying to create mailing lists for different product groups on my site. 
I therefore want import clients  via .csv files I can create from my offsite data base.
However I want these clients to come in tagged with different interests so I can then send email campaign to specific tagged groups.  No idea where that is covered. 
Also segments - I seem somehow to have created one - but no idea how to create another or how to add new 'clients' to existing or new  segment.
So need comprehensive Tag and Segment Client/customer advice.