I can be ranked on any page of Google, or I must be at the first page?

If you can rank on the first page within 7 days, that'll be great. But any page of Google is fine. We will choose winners according to a position like - your post is on the 5th page, another participant's post on the 7th page. Then you'll be the winner.

If nobody can make it to Google's search page, we will select the winner according to post quality.
[Tips: Create backlinks from Sites like Quora/Medium to index/rank your content fast!]

And from which location it will be checked

I use Google USA, but it doesn't matter as almost every location shows almost the same result. And we will be using SERP tools as well to check the ranking. So, don't worry about that. :)