ayyy ok
The first thing you are showing is build kinda monotone, maybe you could add some more darker blocks such as dark prismarine etc. And add some other grey blocks like (polished) andesite, or regular stone.
Also, the walls are really flat, you could ass some more depth to them by making some parts come out more, like this:
View attachment 158533
For the cave,
Isn't that bad, just try and make the tunnel a little more cavy, currently its just a straight tunnel.
For the island, the underside is really the same its just a (kinda) piramid upside down, try and make it look more natural by making the 'piramid' a lot longer. Like this:
View attachment 158534
(I made this really quick, so looks weird)
And add some layers of different blocks into the island: ex
1. Sand
2. Sandstone
3. Sandstone
4. Terracotta
5. etc.
The top of the island is really empty, try and add some landscape to the top aswell, just a couple of extra sand layers.
There are some extra small points, but they dont really matter so yeah. For one of your first builds definitly not bad at all.