In general:

1. clear out all monsters close to the boss area
2. put down campfires to prevent them from respawning during the fight
3. prepare the boss arena to work in your favor using ax, pick axe, hoe, or hammer to improve the terrain or give shelter
4. make sure you have a spawn point or portal close by, but not somewhere that the boss will destroy it, so you can get back if you die
5. if you can't get on youtube, read the wiki, so you know the boss's resistances and weaknesses because some weapons work better than others https://valheim.fandom.com/wiki/Valheim_Wiki