Question about order and some words.

Hi.
I'm studying English and have some questions for you.

Or, ...have some questions to ask you.

1. When I order something in a pub or coffee shop, I usually say "Can I have Fish&Chips?" or "Can I have an Americano, grande size?"
In this way I order something by using "Can I have blah blah~"
and I have tried to listen other people who ordering something, but they speak too fast for me to catch the words.
I'd like to know how do YOU say it when in ordering something.

Can I have... is perfectly fine. The other polite options are May I have... and Could I have...?

More casual ways of asking are I'll have..., or Just a... for me.

If you've been given a few options by the person serving you, another way of making your request is using I'll go for the...

Remember to always end the request with please! This is basic politeness.

2. I'm practising my listening by watching a drama named The Big Bang Theory recently. I heard "If by holy smokes, you mean derivative restatement of the kind of stuff blah blah~ " but I can't understand "If by". What does "If by~" means??
Could you make some other sentences using this phrase??

Here's how it works. You do need more punctuation to make it clear.

"If by 'Holy Smokes', you mean a derivative restatement of the kind of stuff you could find scribbled on the wall of any men's room at MIT, sure."  That's the full quote.

It works like this: "If, by you saying 'Holy Smokes', you mean...." ie. "You said 'Holy Smokes'. Based on that, if you mean... etc etc" 

Hope that helps. ;)