what should i do with 16 lingots that i have ? in the store?
You could buy a Streak Freeze and keep the remainder until you have enough for another Streak Freeze.
You can buy the items you see in the app(attires don't add anything to the course) and there's also a "speed test" available for buying via the site. You can give them to users with comments you like, too. I personally don't like to use the streak freeze, to say I did all this streak without freezes! And "Double or nothing" is a good thing to keep you on streak.
I use them up by going through the discussions and leaving prizes/lingots for people who leave really beneficial comments or posts :) this encourages a beneficial discussion section