Finding the vaulting pole
The vaulting pole is the first item that you will need. This is something you will likely have already seen in trailers, as it's a simple pole you use to jump over bodies of water.
On day one, you probably won't be able to craft a vaulting pole. Don't worry though, Tom Nook will gift you with a vaulting pole DIY recipe once you have completed most of your tasks - you should get the vaulting pole on your second day on the island.
Alternatively, you could always have a friend take you to their island to purchase a vaulting pole DIY recipe, or they could drop you a pole for free…
Building a bridge
The vaulting pole is nice, but you can't use it forever, right? It's too awkward to keep swapping your tools around. That's why you need a bridge.
Once the full Resident Services Center is constructed you will be able to construct bridges. Just talk to Tom Nook inside, and he willl give you what you need to mark out a location to construct the bridge.
You won't be able to place a bridge anywhere, and it'll be difficult to find an ideal space without terraforming, but keep trying and you'll find a place. Then you just need to give Lloid the gyroid enough bells to construct it, and wait a day for construction to complete.
Find the ladder
The ladder is necessary to climb up cliffs, which is basically a whole third of your island locked away until you get what you need.
Just like the vaulting pole, you can have Tom Nook gift you a DIY recipe for the ladder, but it takes much longer than the vaulting pole did, and you might be completing tasks for the tanuki leader for several days, gathering materials and placing down buildings.
Which is why in this case more than the vaulting pole, I suggest having a friend who has the game to gift you a ladder, or allow you to buy a ladder recipe from their store.
Building an incline
And just like the vaulting pole, it becomes tiring swapping your tools around all the time, especially when you have two dedicated just to moving around the town.
Which is why you can build an incline. Simply find a cliff in town that you would like to mount more easily, talk to Tom Nook, and you can be given an incline layout to place wherever you wish - and wherever it'll fit.
This is very similar to the bridges, as you will have to feed Lloid the gyroid with bells before it is complete - but once it is done, you and your villagers will be moving around much more easily.