How do you know which word for the english "for" to use, because you can use"for" or "til" to mean "for", right?
"Til" is usually used when something is for someone or is used to do something.
In this sentence, I actually think "I am saving the apple until tomorrow" would be a more direct, albeit less idiomatic, translation.
Prepositions are the worst. :P
Why is it not “jeg redder æblet til I morgen”?
You do not use "redder" in this case. It is used if you (e.g) save someones life.
"Jeg redder ablet" means that the apple is in danger and you saved it from that.