This repo contains the source code for the my Tilemap Collision tutorial.
You can find a written version here.
Obviously, this tutorial is built using GBDK-2020, be sure to download it and become familiar with it beforehand.
This rep compiles two files: TilemapCollision.gb, and TilemapCollision_SinglePoint.gb. The former does tilemap collision against a 16x16 square, the latter does tilemap collision against a single point. For the 16x16 square, more collision detection tests are needed to handle the players size.
If you need help, you can: