Correct way to order drinks at a bar?

Let's say that I want my own mix drink. Should I just state the liquor and say, "with oj"? What if I want more OJ in it? If I just say "rum and coke" will they give it to me in a standard gl, or do I have to tell them before hand? For example, "I would like a short rum and coke." I'm new to this and I don't want to look like an idiot.