To say "I miss you" in Portuguese, you'll say "Sinto saudades!" Saudades is prounounced "sau-da-dish".
"Saudade" is a verb in Portuguese that means much more than the English word "miss." Saudade means having an intense yearning and longing for another person or place. It is usually a romantic word.
A phrase that would go along with "I miss you" is "I always think of you." To say that, you'll say "Penso sempre em você."