Minecraft wiki entry for Player_versus_Player, associated text: Start gathering immediately. Do them in this order;
-Get wood.
-Make a bed to set your new spawn point (unless your server is customized).
-Make chests and protect them.
-Build a farm.
-Find coal and iron.
-Look for diamond. Layer 12 eye position is the best, but look for caves especially.
-Mine obsidian and reinforce the walls of your base.
-Make a portal to the Nether in order to find blazes and nether wart.
-Once you have a blaze rod or two, start hunting for Endermen.
-Make an ender chest.
-Start brewing potions.
-Find leather and sugar canes.
-Make an enchantment table if you have the tools to and keep it well concealed.
-Make an anvil to combine enchantments.
-Enchant your diamond armor and sword.
-Tip: Enchant Some leather armor with Protection IV if possible and a stone or wooden sword with extreme enchantments (stone axe with Sharpness V works wonders on 1.9+). This will trick players into thinking "That player is terrible and doesn't even have good armor". Then sneak and follow them (use an invisibility potion if you have one), and they may end up revealing a base or hidden valuables. If you know you can beat them then go for it. Big tip: use potions always!