As Shogo999 mentioned, you’ll be diving into a ton of material to learn. Most beginners or people without a large budget start with blender or Maya LT. (I use Maya LT as it is quite affordable and the tools you learn to use is quite translatable to Maya if you ever make the switch.)
I would make the argument that it doesn’t really matter where you begin in terms of asset building, HOWEVER, I would highly recommend that you create a demo piece to test all the basics of your game before starting to make like 500 different levels for your game.
I do agree you should start with learning to create static meshes (game assets that do not undergo deformations like characters) for the basics. If you’re still trying to make the decision, here is the minimum for what you’ll need to learn to create a character by yourself:
In your 3D program (Blender, Maya):
- Modeling - This is the most obvious part. You need a structure that your player recognizes as a person or character. Try googling low-poly character modeling tutorials for the program of your choice to get started.
- Rigging - Setting up the “Skeleton” of your model. Long story short, you’re setting up joints that have influence on parts of your model. When those joints move (definitely look into control objects), your model deforms. An adequate “rig” is necessary so that you can animate the model in the future.
- Skinning - This is a whole process where you define exactly how much influence each joint in your skeleton has on your model.
- Animation - Using the rig you made, an animator (or you) will now use it to make animations that can then be called in your game. This step is much easier if your rig is made to accommodate all the animations you’ll need for the character.
Then you still need to export all these assets to UE4 and do more stuff to them.
In my experience, creating an environment is not necessarily “easier” but definitely more intuitive than making character assets. Many environments can be made with mostly static assets to get started. If anything, you should start with the scope and concept of your game because a well planned concept will define exactly what you need to make.
For example, when you’re animating your character, how do you know you’ll even need climb animations if there is nothing to climb in your environment? On the flip side, how do you know you’ll need to make the environment climbable if you don’t know whether your character can climb?
If you start by jotting out the essentials for what you’ll need, it’ll be a little easier to define where to start. To be honest, you’ll probably be going back and forth anyway if you’re going solo.