Answer:
Du bist ein sehr netter Junge.
Explanation:
In this sentence, you are using the nominative case meaning that you'd remove the 'en' from words like 'einen' when referring to masculine objects. As a result, you'd use 'ein' instead of words like 'eine' which is feminine. You'd know if you're using the nominative case because the boy is the subject in this sentence. (Who is very nice? You.)
When you are using adjectives (words used to describe) before the object, you would the gendered form. Because 'Junge' is masculine, you'd add 'er' to the end of your adjectives. This makes 'nett' turn into 'netter'.
This is me nit-picking here:
In German, nouns (person, place, or thing) are always capitalized. 'Junge', or boy, is referring to a person so that word is capitalized. (Also, you should add periods to the end of your sentences too.)