From this page of the Valheim game guide you will learn where to find tin ore deposits.
Tin is one of the first metals you will get to mine during the game. When combined with copper ore, it produces bronze. This is a crucial step in the technological development process. From this alloy you will be able to smelt better tools and more powerful weapons.
Tin deposits can be found in the Black Forest. However, you won't be able to find it just anywhere. This raw material can only found in the vicinity of water bodies. You won't require anything more than the simplest antler pickaxe to extract it.