You can get Consumer Key from your Developer Account. If you already have a developer account, just go to the “Keys and tokens” section from the Apps page, there you will find the “Consumer API Key“.
Reference
You can find out how to create a Twitter App in the following article:
https://jkdevstudio.com/documentations/maniel-multi-concept-blog-magazine-wordpress-theme/tips-tricks/twitter-api/how-to-create-a-twitter-app/