My first custom terrain

quite apparent you did this in game with world edit or voxel sniper, which isnt the best way to go for realistic terrain or large scale terrain because its long and tedious, as well as slow and hard to mold. A few suggestions: 1) If you do use world edit for terrain, go back over it with the /brush smooth (Radius) tool so you can make it at least look somewhat more realistic and 2) Dont edit terrain in game, or with mc edit. Use world painter or if you can afford it world machine

But thats just my two cents

You should have used any form of smoothing on your terrain, that would already make the terrain more "realistic"

If this is your first time making a terrain. Your on the right track Because this looks fine for a start.