Hi and welcome to SaleHoo, Marius - it's great having you with us :)
Once you find a dropshipper that suits your needs, YES its is best you initiate contact through email or phone. If the dropshipper requires that you register to join then best that you register an account with them. Oftentimes registration is free.
Tax ID is the equivalent of VAT ID in Norway, so if you are required by a supplier to provide a Tax ID then you can use your VAT ID instead.
You might find this helpful - Link hidden: Login to view
All the best!
PS - Do add a profile picture the next time you get a chance - Thanks :)