If you would like to activate your amiibo figure in your game, please head to the amiibo building in the Peach Castle area. This building is just behind the castle, so you will have to walk around it.
Alternatively, you can just take the pipe at the entrance of the castle and it will directly take you to the amiibo building.
Once you enter the building, a new screen will display. Here is the list of amiibo that you can activate: Mario, Peach, Yoshi and Luigi. To add one amiibo in your collection, simply press A.
You will now be prompted to hold the amiibo to the NFC touchpoint.
Then, you will receive a confirmation for your new reward. Here is an example for the Mario figure:
Your amiibo should now be available in your own in-game collection.
Be aware that 4 new weapons can be unlocked through this feature and that you should receive one new exclusive weapon per character. Please note that every figure of the same character (i.e. Mario Gold, Mario Silver or Super Mario) will unlock the same weapon. For more information about the content, please visit this article.
If you want to know more about amiibo figures, visit the Nintendo's article.
If you are unsure where to find the reward you unlocked, please check out this article.