Play "Pixelmon" by choosing a starter Pokemon, pressing the "r" button to throw the Pokemon out into the world, and use the menu at the bottom of the screen to fight battles. Gameplay in "Pixelmon" generally consists of catching Pokemon, fighting with them, healing them, finding and using fossils, and evolving Pokemon with elemental stones.
Catching Pokemon requires Poke balls. To craft Poke balls, gather apricorns from apricorn trees scattered throughout the world. Attacking the trees makes them shatter and reduces the item yield to mere wood, so right-click the tree instead to collect the apricorn. This leaves the apricorn trees alive and able to produce more apricorns for players to collect at a later time. After gathering three apricorns, cook them in a furnace, and use a crafting table to create a Poke ball disc.
Next create iron and aluminum discs using iron and aluminum ingots. Make an anvil with eight iron ingots and a hammer with five wooden boxes and two wooden planks. Right-click the anvil with a Poke ball disc, and hold down left-click on the anvil with your hammer. Repeat with the other discs, and place them appropriately into the crafting grid to create a Poke ball. Use the Poke ball in battle to attempt to capture a Pokemon you're fighting.