First, a quick review of prepositions:
Prepositions help clarify the relationship between two aspects of a sentence, usually in terms of location, time, or quality. Examples:
The boy caught the frisbee in his hand.
(The preposition in shows the relationship among the frisbee, the verb caught, and the boy's hand...in terms of location.)
We watched The Walking Dead after dinner.
(The preposition after shows the relationship between the The Walking Dead, the verb "watched," and dinner...in terms of time.)
I bought a Jeep with great off-road capabilities.
(The preposition with shows the relationship between the Jeep and the capabilities...in terms of quality.)
Prepositional phrases, then, include the preposition and its object.
You can improve your grammar by avoiding one common stylistic error. It isn't really a terrible one, because doing it does not (generally) create grammatical disasters...just stylistic issues. Here it is:
Don't begin sentences with prepositional phrases. People make this common mistake when they're using an adverbial prepositional phrase. All you have to do to fix this is move the prepositional phrase after the verb that it modifies.
NOT AS GOOD: After dinner we went to the movies.
BETTER: We went to the movies after dinner.
NOT AS GOOD: In high school, I read many books.
BETTER: I read many books in high school.
Why is it better to move the phrase after the verb? Because it moves the subject and verb closer to the front of the sentence, which strengthens and clarifies the language. Remember: prepositional phrases are modifiers, so they are less important than major elements like subjects and verbs.