Yeah, you definitely have grammatical mistakes and some poorly-constructed sentences, but Rule probably covered it all so I won't bother rehashing.
I don't know whether this is the only essay you'll be sending, but regardless, here are some major points that you should communicate in college essays:
1. the type of person you are (you did this well)
2. your skills, experience, hobbies, qualifications: things to impress them, things to differentiate you from others (you made no mention of this)
3. the strong points of their school that interest you, demonstrate knowledge of their school (you made no mention of this)
What it comes down to is that you want to impress them, and you only covered 1 out of 3 of these points. Vanity is fine, up to the point of obnoxiousness, and you should have your school adviser who handles college apps be the judge of this.
I don't know much about you, hence not much about your skills/experience, but I think I can assume there's a good chance you've meddled in comp sci and programming. You can likely impress them with this, as well as convince them that it translates into a logical mind, enthusiasm for learning, and an ability for independent research -- and that these skills will be very useful in your academic career.
Oh, and the 400 word limit is all the more reason to be succinct and economic.