Congratulations

[Samara]

How do I say at a weding: Congratulations & a happy live together

[Kates]

Congratulations is "Omedetou". To say specifically for a wedding, say:
Go-kekkon omedetou gozaimasu. (ご結婚おめでとうございます。）

And a real fancy way to say 'I wish/pray you both will be happy from my heart' is:
O-futari no shiawase wo kokoro yori o-inorishite-orimasu. (お二人の幸せを心よりお祈りしております。）

(Luckily, I got married a few months back and got a letter from a Japanese friend! XD These were wishes in her letter to me, so I know they will serve you well. ^_^ Congrats to your friends!)