Hello! I need a help!
I had a task to write a application letter. My teacher marked out my mistakes, but didn't correct them, having told: "Go on a forum and ask native speakers."
I ask you, help me to correct my mistakes.
I considered and came to a conclusion that I have a good experience in work with children, so I decide to apply for this job.
As I training in a gym, I could become the sports leader.
As to the work beginning, you specified that work begins since June 1. With the fact that I must study at college till 15 June, I could start to work only since 16 June.
I really hope that you employ me on this job.

That is mark out red color are my mistakes. Please, help me to correct them.