If you have enough karma, you'll see a "flag" link under the submission title. I think the threshold is 20, (so you'd need 21) though I may be mistaken.

As you're new, I'd be hesitant to flag too many submissions, even after you have enough karma. Take the time to get to know the community more, and its standards.

You may want to review this as well:

https://news.ycombinator.com/newswelcome.html

If you need help getting used to the community feel free to ping me.