How do I make a waypoint in Minecraft? I know you can but I can't figure out how. I don't mean things like beacons or high towers of blocks, like those signs on Stampy and Squids' race to the moon video. Please tell me how to do that or make my world UN-infinite.
You can't in Vanilla minecraft. Rei's minimap allows you to make waypoints, and also includes a minimap.
For waypoints, you need client-side mod for minimap. You can do it by installing Forge or Liteloader, depends on your minimap (or ingame map) choose. How to install these you can get on the websites of corresponding projects. (Rei's minimap, Zan's minimap, voxelmap, journeymap, mapwriter, and more....)
For limited world, you need some server-side work. Known solution is to have the server of Bukkit/Spigot/Cauldron type, then installing plugins. Here you search for WorldBorder. It can setup inpassable border in any diameter, circle or square. For each dimension separate one. Note: With the server already installed, you can also install Dynamp plugin, then you will be able to see the map in your browser, however no waypoints in-game with it.