If you are playing Minecraft for the first time, we suggest that read this short guide on how to craft weapons and tools to survive against other players and monsters.
The first thing you should do when you spawn in a new world is to look for trees. Hold down your left mouse button to punch the trees until it breaks into a block of wood. Just stand next to it and you will collect it into your inventory. Keep doing this until you've got about 5 or 6 blocks.
Make a Crafting Table
Open your inventory (press "E"), click on the stacks of wooden blocks, and drop it into your 2x2 crafting grid. On the right side you'll notice that it will craft into 4 wooden planks per block. Hold down SHIFT and then click the wooden planks to instantly craft them and put them all into your inventory. Next, place 4 wooden planks in a square formation into your 2x2 crafting grid. You should have at least one wooden plank in each grid. You should now see a crafting table appear— click on it and put it into one of the slots on your hotbar.
Select your crafting table from your hotbar by pressing the number that corresponds to the slot you placed it in (1-9). Then, place the crafting table down by right-clicking an area on the ground. Next, right-click the crafting table and you will have access to a 3x3 grid, which is required to craft advanced tools and weapons. Luckily for you, you can make a sword and a pickaxe with just wooden planks. Swords are your best choice for dealing damage, and a pickaxe is needed to mine better materials to craft better tools and weapons, among other useful things. First make 4 sticks by placing 2 wooden planks in a horizontal position in your crafting table. Place the sticks in your inventory and take anything out of your crafting table. To make a sword, place 1 stick and 2 wooden planks in a horizontal position. For a pickaxe, place three wooden planks horizontally in the first three squares, and then two sticks horizontally below it in the center squares.
You can now choose to hunt down other players, try to find animals to kill for food, or head into a cave to mine stone blocks to upgrade your sword and pickaxe. If you are playing PvP (player vs player), you should try to come from behind and critical strike them by jumping and then hitting them on the way down. Do this until they die or turn around and fight you. If you don't have the element of surprise, try to strafe and hit them from an angle while right-clicking to block their hits. You have to repeatedly strafe left and right or circle them in order to do an effective combination.
If searching for food, you can kill cows, pigs, and chickens and then craft a furnace to cook them. You will need to mine stone with your wooden pickaxe, upgrade it to a stone pickaxe, mine 8 stone blocks, and then craft a furnace by placing those 8 stone blocks in a circle formation inside your crafting table. You can use the furnace the same way as a crafting table; just place it on the ground and right-click it. You will also need coal or wooden planks to use as fuel for the furnace. Place the meat you want to cook in the top position and the fuel in the bottom position to begin cooking.
If you are a high risk high reward type of player, look for a cave or ravine and try to mine iron ores, which you can smelt with a furnace into iron ingots. Iron ingots are great for upgrading your wooden or stone sword to iron, and for crafting iron armor (second strongest weapon and armor in the game). Diamond is the strongest, but it will take a while to find any diamond ores to mine.
For more resources, you can check out the Minecraft Wiki.