It's always fun to try speaking a foreign language, even though the locals will probably understand your English a lot better than your Icelandic...
You may be aware of the fact that most Icelandic people speak fluent English. I hardly doubt that you will encounter any difficulties communicating with the locals, as long as you speak English.
I've previously written a short article about the most useful phrases to know in Icelandic, but now we're moving on to restaurants.
If you are confused about the pronounciation, you can read my article about the “special letters” in Icelandic or check out tutorials on Youtube.