There are polite phrases for many situations in life. For example, if someone gives you a cookie, the appropriate thing to do is say, "Thank you." If someone is having a new baby, the appropriate thing is to congratulate them.
But what about when your friend announces that he/she has just started a relationship with someone special? Society and etiquette books don't seem to have an approved, polite phrase to say in that situation, so you're on your own.
Just to help you out, here are some phrases that are completely appropriate in some situations, but NOT when your friend tells you they're seeing someone...
"Knock yourself out!"
"I love you, too."
"I'm sorry for your loss."
Just a word to the wise.