This page of the Valheim game guide provides instructions how to increase your character's weight capacity.
The weight capacity of each character in Valheim has been set to 300 by default. This may seem like enough at first. However, after your first day in the game, it will turn out that it's not sufficient at all since one wood stack (50 pieces) weighs as much as 100. Fortunately, there is a way to increase the weight capacity of your character.
In the Black Forest area you can meet a merchant, Haldor. Unfortunately, finding this NPC will not be as easy as it seems, because Haldor will appear in a completely random place on the map and usually it will not be the area of the island where the player starts the game. Once you are near Haldor, a coin sack icon will appear on the minimap.
Haldor offers a unique belt, Megingjord, which increases the character's weight capacity from 300 to 450 and which you can buy for 950 coins. Megingjord must be equipped the same way as any other clothing items.