Welcome to the forum.
There is multiple ways to do this:
1. field based:
A newly imported person will also gat a certain tag, a field or any attribute. This information qualifies the person to be a member of a segment. (contact has a certail tag) The segment will be the source of your welcome campaign. Campaign steps are the emails.
2. Segment based:
You already place a person to a segment, once you import. As above - the segment can be the source of the campaign, and emails are sent out.
The difference is in how you want to handle the contacts in the segment later. If you want to move them out from this segment by campaign action, or just by removing the qualifying attribute from them.
But anyway, both ways are fine, you can change this later.