There is no right answer to what product you should build; that will be different from person-to-person, your passion, your skills etc.
Here is a quick list of ways to generate ideas:
1. Talk to a teacher (or figure out what you can train others to do!)
2. Interview people. Ask what are the top 3 things they are facing today that they'd LOVE someone to solve.
3. Find something massively popular and sell it better.
4. Piggyback off something massively popular by selling a side-product/service.
5. Craigslist for what people are wanting.
6. Check marketplaces (eBay, Amazon, Etsy) for what is hot.
7. Ask Friends/Family for what they would want.
8. Go to a coffee shop and do some recon with the locals.
9. Check out online, newspaper, and magazine ads
10. Look at successful crowdfunding campaigns and compare to what is unsuccessful. Check who their backers are, see what those creators backed, etc.
11. Find out on Google what people hate
12. Google "Business ideas"
13. What'd you do last weekend?
14. What did you buy in last week?
As for validating it, ask your target audience to buy it from you before you build. Until you have money in hand, you have not validated anything. Though you should proceed with caution, Reddit can be very powerful in doing this.