Well, I know a little elvish, and I can help you!
You can say
"Dina!" which means "be quiet/be silent" but I much prefer other insults...here are a few of my favorites:
"Auta miqula orqu!" (go kiss an orc!)
"Lle holma ve'edan!" (you smell like a human!)
"Lasta lalaithamin!" (listen to my laughter!)
"Dolle naa lost!" (your head is empty!)
And the best one is...
"Llie n'vanima ar' lle atara lanneina!"
(you're ugly and your mother dresses you funny!)
If you want some more, check out this site, it has great info, but I think some of it is a mix of Sindarin and Quenya, but it stays pretty close and it's organized really well:
Elven Phrasesnull