How do the first users of new Stack Exchange sites get reputation? You need 15 rep before you can upvote, but if nobody has rep, nobody can upvote. And if nobody can upvote, nobody can gain rep.
As long as you have an account on a sister site with at least 200 rep, you can get an initial 100 rep from linking your accounts. As long as this is true, that will never be a problem.
If you were starting from scratch without any sister sites, you'd want to remove the restriction on required rep to upvote for at least the first week or so, or have an admin/moderator give out the initial upvotes, or maybe have a "beta" period where any participants get an automatic +100.
But even if you didn't, you can still accept an answer to a question, which gives you +2 and the answerer +15.
This has been taken care of now by allowing users to associate their accounts to other accounts they may already have on other sites, and doing so will give them an immediate +100 boost to their new profile.
A StackExchange site, be it part of the SO/SF/SU 'family' or the hosted commercial brethren without sisters sites (and bonus), is about more than upvotes and scores. There is a significant 'community' dimension, and that needs building. To that end, these sites start out with some moderators and they can upvote whoever they please, which can kick start the community and resolves the apparent vote-score paradox. Receive two upvotes and a user is in the game...
- You make a question;
- People Answer your question;
- You select in one of them as the right one (don't need to upvote);
- The user who made that answer gets 10 points and you get 2 points for accept.