With the Caves and Cliffs Update Part 2 in Minecraft, world generation has been altered significantly. Not only are the caverns you will dig through be much grander in scope, but now the spawn criteria for resources have been changed since the world height, and depth limits have been expanded. If you are playing past Minecraft version 1.18, here is the best level to go hunting for Diamonds.
The best Y level to find Diamonds at in Minecraft 1.18 is now between -53 and -58. Overall, Diamonds can generate between Y levels 16 and -64, but between -59 and -64 will mostly have Bedrock. The closer to that bottom number you get, the more likely you will find a chunk of Diamond that are generated together. These chunks can spawn with multiple Diamonds, so be sure to check all around that chunk. You will be able to find Diamond above that level up to Y 16, just not as common as lower in the ground.
If you are looking to find your Y coordinate, you will need to set the game to tell it to you. On Bedrock, pull up Settings, and in the Game section, you will need to switch Show Coordinates to on (cheats will need to be enabled). On Java, press F3, and your coordinates will be on display near the center of the screen.