why did God allow evil to exist?
Because He is just. Evil actually does exist, and for Him to create w/o having that be a part would not be just.
why put the tree of knowledge in the garden when it can mess up his plans?
All of evil, was confined to one tree. One. That was His plan. That you conflate this with somehow "messing up his plans" is to demonstrate your lack of understanding.
Further, we can minimize all evil to one tree in our own lives today. (This is the way of the Cross)
when he made the earth he saw that it was good so why put something which would release evil AND banish satan to earth?! did he want them to get tempted or something? the bible made adam and eve sound like ten year olds so it bothers me why he sent satan to earth.
There is nothing in the text to suggest they are 10 year olds. Where do you get that idea?
will there be free will in heaven ?
We don't actually know, but from everything that is revealed to us so far it seems reasonable to conclude that our responsibilities will only increase int he next life.
if so, why didn't God just design earth and heaven that way so that we wouldn't have to through all this trouble?
This is a very good question for you to be asking at your age. It is also a very important matter to SETTLE. The fact is, this is God's plan. This life is not heaven, and it involves struggle, pain, suffering, and death. You have no choice but to get used to it. Might as well embrace the Cross.
also why did God get angry at A&E if they did not posses the knowledge of good and bad? surely they did not think not following Gods instruction was wrong right?
This is all nothing but misunderstanding on your part. Whether this was taught wrongly to you, or you came up with this yourself I don't know.
In a first-hand experiential way, A & E "knew" good; they were surrounded by it. They only "knew" death based on what God told them; i.e., Faith. Just like you and I
The only way they could "know" evil was to actually lose, rather than gain anything. By being kicked out of Eden, suddenly they "knew" evil! That was a loss, not a gain. That's what happens every time we violate God's plans and purposes. Every. Time.
Notice that what I'm saying holds true every bit as much regardless of the story is allegory or historical fact.
since he will abolish sinful nature in heaven why not just do that in the first place?
Not His plan. Doesn't achieve His purposes.
and why do we have to inherit sin when we weren't the one who sinned? i dont think its fair for us to suffer the consequences of those two idiots.
First off, watch your mouth as you have fared no better. Next, you are reading it wrong. This may well have been taught to you. We don't inherent condemnation from God, neither do we need to. You and I have fallen short of the glory of God all on our own, w/o needing anyone else's help.