It's 'einen' because 'Tisch' is masculine, and in the accusative case (i.e. direct object).
Yeah, it can be confusing in the beginning. But keep going!
Sie can mean "she", "they", "you (formal singular) and "you (formal plural)". And that's just in nominative! Normally sie can be narrowed down to "she" or "they" as you've already figured out, but at the beginning of a sentence you can't be sure if it has a capital because it's polite or because it's just at the beginning of a sentence. But anyway, remember that "she" uses a different verb ending to "they" (which also uses the same verb ending as the formal forms of "you"):
Sie hat - "She has"
Sie haben - "They have" or "You have" if I'm being formal
It does not accept "desk" as Tisch. I wrote: "They have a desk." Any explanation would be welcomed.