You are very pretty: Anatawa totemo kireidato omoimasu.
You have beautiful eyes: Kireina me o shiteimasune.
These are my suggestions. I think it would sound more sincere by adding "omoimasu", and "kirei" sounds better than "utsukushii" which may sound a bit too much.
These are very English expressions, though. Most Japanese men would just say "Kawaiine", because directly praising a girl's appearance is not a very Japanese thing, and if a man says "kimiwa totemo kireida" or "kimino mewa utsukushii", most Japanese girls would take it as a joke, and start giggling. I hope it works for you. Good luck!