Find a lava source
Build a bucket from 3 iron ingots
Collect water in a bucket
Pour water on lava source blocks to create obsidian
Mine obsidian with a diamond or netherite pickaxe
Use a Silk Touch pickaxe if you want to collect the obsidian block directly
Find obsidian naturally in ruined portals, Nether portals, and some chests
Trade with piglins for a chance to get obsidian
Use a lava and water setup to generate more obsidian safely