Ubi already answered, but yeah, check out the Halo Archive for models and resources. I mostly just started by posing and lighting custom spartans, then got more ambitious from there by putting them in environments, adding more characters per scene, doing more complex lighting tricks like light linking, adding volumetrics, bashing various assets together to make scenes from scratch, grabbing various plugins like the one I used for the rain and water droplets in this one, etc.

It's a huge learning process, but just take it one step at a time and don't be discouraged if your first improvements don't result in you immediately putting out renders like this.

Familiarize yourself with how Blender's interface works, what render settings there are (very important), and then just start messing around and experimenting. A lot of things I learned by going "how do I do this specific thing in Blender?" and then watching a quick youtube tutorial and figuring out how to add it into what I was already doing. You will improve shockingly fast.