This is going to seem overly simplistic, but... Practice!

1) Find designs that you love, copy them pixel for pixel

2) Start mixing and matching elements from different interfaces that you find.

3) You'll start to see repeated patterns, get good at connecting these patterns together into new interfaces.

4) After a while of this, you'll see that you don't need to do as much research and direct copying anymore.

5) Congrats! You are a junior designer, keep practicing :)