Someone that I don't know very well likes me, but I don't like them back. I'm scared what will happen if I reject them.
Someone that I don't know very well has an unrequited crush on me. I'm scared and don't know what to do.
Someone that I don't know very well likes me, but I don't like them back.は、「一方的」みたいな訳語がないですが、一方的に好意を寄せられるときに一番使われていると思います。themの代わりに、男の子ならhimにして、女の子ならherにしますね。
Someone that I don't know very well has an unrequited crush on me. のunrequitedは恋の話で一方的と言う意味で使われています。have a crush on someoneは、付き合いたいと言う気持ちを表しますね。
I'm scared what will happen if I reject them. 誘いを断った時にどうなるか(悪いことになるか)がこわい
I'm scared and don't know what to do.怖くてどうするか分からなくて困っている
のどちらでも使えますね。
ご参考になれば幸いです