Translation:My friend is not good at singing songs.
No, simplified chinese letters are totally different things. However, there are few cases for hand writing to simplify some Kanjis. You may see simplified 門 and 第 in hand writing. Others, not so much.
"de wa arimasen" is best just memorized as a phrase meaning "is not." Its contraction, "ja arimasen," has the same meaning. The explanation for WHY "de wa" is used here is somewhat esoteric, but derives from the verb to be, "de arimasu," though "desu" is far more common in modern Japanese.
「わたしはうたがじょうずな友だちがいません」 This sentence requires knowledge of a couple more advanced concepts. To expound a bit, 「上手」is a so-called "な-adjective," meaning that when it precedes and directly modifies a noun, you must include "な." As in, "じょうずな友だち".
Your sentence includes the relative pronoun "who," which Japanese does not have. Instead, in Japanese you simply precede the noun with the whole modifying phrase.
For example, the English phrase "the man who wears a hat" would in Japanese be something more to the effect of "wears-a-hat man" （ぼうしをかぶる男の人）.
So for your example, you'd have to say something to the effect of, "I don't have good-at-singing friends."
Edit: Note also that you don't "have" friends in Japanese. They exist (or don't). ”友だち が います／いません"