"A salad, please."
Translation:Una ensalada, por favor.
Un is used for masculine nouns, una for feminine. Rule of thumb (that of course doesn't always apply): If it ends in "a" it's probably feminine, if it ends in "o" it's probably masculine. But not all nouns end in A or O obviously and those you just have to memorize. For the most part the "O" and "A" endings being masculine and feminine respectively are correct; I'm not coming up with any exceptions to the rule though I'm sure there are a few.
Yes, but be aware that it is colloquial and would be inappropriate in certain circumstances. I wouldn't use it, for example, in any circumstance where I felt I should use usted instead of tú.
FULL DISCLOSURE: Native English speaker - US, Southern Appalachian dialect. Other uses of English may vary. Advice about Spanish should be taken with a grain of salt.