This is a lesson I did specifically for a student aged 13-14, who wanted to know how he could improve his writing level. We took a story that he had written for homework using pathetic fallacy.
Most of his sentences started with a noun or pronoun. This is the most common way to start a sentence, and is useful. But if every sentence starts this way, it can reduce your grade.
I I’m, I’d, I’ll
Name of a thing or person
To help us, we used this post: how to vary sentences. Then, I wrote up some examples of how he could start his sentences in different ways. Each one is levelled, or graded. Please note, though - it's hard to grade a sentence in isolation.
If you want to try this at home, take care that you don’t make every single sentence complex. Leave about half (or more), simple. Make some, very short. Or you can even use fragments. Like this.
 The Prison, Fox River, was cold and dark. [student's original sentence]
Inside Fox River Prison, it was cold and dark.
Inside Fox River Prison was dark, cold and hard.
Damp, dark cold seeped through the walls of Fox River Prison.
Inside Fox River Prison was dark and cold with guards as miserable as the inmates.
 It was raining outside and I could hear the thunder booming too. [student's original sentence]
Outside, rain and thunder thrashed the walls.
Suddenly, thunder smashed through the rain.
Thunder and rain cracked around the walls. (still starts with a noun, but has a more interesting and unusual use of the verb)
 It was as if the sky as as angry as me. [student's original sentence]
The sky, and I, were furious.
Outside, the sky raged; inside, I seethed under blacker clouds.