You can place custom patterns down, or eventually you will unlock a feature that lets you place down paths from the game itself (ones that have the grassy edges and also show up on the map).
This happens when: (Spoilers, do not click unless you really want to know)
After K.K. Slider gives his first concert on your island, which is basically when you have progressed through the main game tasks. Keep talking to Tom Nook to find out what to do next to make progress on that!