Buried Treasure spawns in beach biomes randomly and are easiest to find using a buried treasure map which can be obtained from a shipwreck or ocean ruins. A useful thing to know is that buried treasure always spawns at chunk coordinates 9 ~ 9. This means if you press f3 and next to "Chunk: " you have the first and third coordinates equal to 9 and you are over the X on the map, the treasure will be below you. This can save time since the map isn't perfectly accurate to the block. Buried treasure contains lots of valuable loot, including iron, gold, diamonds, cooked cod/salmon and tnt so can be very useful if it doesn't take too long to find. Once you have obtained this loot, you should be ready to build a nether portal to enter the nether.
See Also
Stronghold Key