'You look pretty today' can be translated as 你今天看起来很漂亮 in Chinese. The pronunciation is 'nǐ jīn tiān kàn qǐ lái hěn piāo liàng'
你 (nǐ) -- you
今天 (jīn tiān) - today
看起来 (kàn qǐ lái) - look
很漂亮 (hěn piāo liàng) - very pretty

You can also use 好漂亮 (hǎo piāo liàng), which also means very pretty.