The process to find your first like will also give you access to everything else you've liked on Instagram. To find your first like, you should go to "Profile," and then select the hamburger menu in the upper right. Then, click on "Your Activity" and select "Interactions." When you select "Interactions," you should see three options: Comments, Likes and Story Replies.

You should select Likes, which will then show you every post you've liked in order from newest to older. You shouldn't have to scroll to the bottom to see your first like, though. Instead, you can go to "Sort by" and select the "Oldest to Newest" option and select Apply. Then, Instagram will show you every post you've liked since joining the platform in chronological order.