Uploading to the portals/0 directory is as simple as using file manager or adding the file/url control to your module.  That part's very easy.

To get csv data into your tables I've used a couple of methods.  Both work fine. 

1. If you want an existing dnn module to do this for you, Bi4ce's ListX module supports CSV imports.
2. If you want to do it all yourself, you'll need to either purchase or develop component to do the csv-sql transformations.  I've used http://www.csvreader.com It works quite well.  There's a .NET demo version that's free and will allow you to see if this suites your needs before you spend any money.