Fishing is one of the easiest ways to obtain resources in Minecraft. You can craft a fishing rod with sticks and string. The easiest way to find string is by killing spiders.
Once you have a fishing rod, there are many different ways to use it, but the easiest way is to just equip it and use it by water.
As said earlier, if you want a better chance of getting top loot from fishing, you can enchant a fishing rod to get the 'Luck Of The Sea' enchantment.