"I drink apple juice."
Translation:Ich trinke Apfelsaft.
Theoretically, the main informations go at the end. From end to beginning: infinitive verb, direct object, indirect object, adverbs, subject. You start at the end and build the sentence back up, as my teacher used to say, "like salmons swimming up a river". ^^
However, since the conjugated verb always takes the second position in the sentence, the first one plays a particular role, useful in longer and/or complex sentences: You put on the first position the 'new' element your sentence is conveying. So in your example, "Ich trinke Apfelsaft" can mean "I am (the one) drinking apple juice", when "Apfelsaft trinke ich..." would be heard as "It is apple juice, that I'm drinking"
Be careful though, this "trick" is only used in longer sentences. There, we would expect another information after the "ich". Such as "in the afternoon, while I drink orange juice in the morning" for example. If your sentence is very short, with only Subj.-Vb-Complement, you will keep the regular order, and simply stress the major word. But since German can have pretty long sentences, and that as I said the regular order puts important stuff at the end, it can be better to sneak the new information at the very beginning. A bit like a film preview, people have a glimpse at what they are going to learn. :)
I don't know if this is a general rule, but I think you pluralize mostly fruits which plurals are formed by adding an "-n": Orange -> Orangensaft, Banane -> Bananensaft, Mandarine -> Mandarinensaft, Tomate -> Tomatensaft, Zitrone -> Zitronensaft. For other plural forms you simply stay with the singular: Apfel (pl. Äpfel) -> Apfelsaft, Pfirsich (pl. Pfirsiche) -> Pfirsichsaft.
But of course there are still some exceptions :) Kirsche (pl. Kirschen) -> Kirschsaft, Heidelbeere (pl. Heidelbeeren) -> Heidelbeersaft.