But how do we know what to do if everything is uncertain? When we realize that feelings are just biochemical reactions and even our own conciousness is just a set of neurons. That is really depressing.

I agree with you. First of all I think objectively speaking nothing has any Value therefore racism cannot be bad or good (in a universal way). But it brings suffering to many people and that I see as wrong, even horrible and I will if I get the chance, do anything to stop it. You got me wrong with the purpose. Purpose itself is, as we both agree, created by thinking beings. A cause is NOT a purpose. And if we say that we've been created by let's say a god and he thought of a purpos for us then this isn't a universal purpose but the idea this god had for us.

This doesn't only apply for existence because every meaning is created by us. And therefore we don't have to explain existence in that discussion.
I'd also like to add that what follows from that is that there's nothing, really nothing we OUGHT to do but we've got infinite things we CAN actually do. And that's the bauty of it. You want to be a humanist then go for it. But you can also be a fascist or racist. Not that I'd like that I hate racism and this stuff but you're free to do whatever you universally speaking. From here as I said begin the roots of philosophy (subjective of course) : What shall we see as good? How shall we act? What are we? Do we even exist?
But in my opinion it all rest on the fact that there's no objective meaning. From there on everything is discussed.

Why do we have to know what existence means? I chose the word "existence" because i didn't want to write down "the meaning of life" because then a lot of people would have said that the meaning of life is reproduction.
By existence I mean "things that are" but the problem here is of course that we don't know what is and what isn't existing. But we don't have to know that to discuss the problem of meaning. We don't know what exists but we know that some things DO in fact exist (I think therefore I am).

But why? Isn't it true that only thinking beings can think of (and therefore create) meaning?

Both of them can only exist if something thinking is "inventing" them. It doesn't even has to be on purpose.

The one thing that goes along with that idea is folowing :
Nothing has any "true" meaning, whatever we do isn't important at all, talking to you now is not important, living is not important , even knowing that all isn't important, shortly : nothing is of real importance. But that raises folloing question : Why are we searching Meaning? why are we doing anything? why am I talking to you and you to me?Why aren't we just all killing ourselves (à la camu) why do we want to know things?And the answer is in my opinion : Because of feelings and thought.
Why am I eating - because I'm hungry. Why am I not killing myself - because I'd rather like to live.
So all our deeds follow from our Feelings and thought?
And that is the exact moment where philosophy and ideology start to apply.
There is no meaning but which invented meaning should we choose to "create" and then follow.

The search for meaning is maybe even the ONE aspect that defines a "higher conciousness". Either way I have had a thought related to the topic of concious beings (I think this word fits better thx T Clark). I don't know if you are known to the thought experiment "Ship of Theseus".
It ask the question when : exactly is the boat ceased of existing when replacing the single parts of it.
There is just one problem : The ship doesn't exist but in our minds. Because a ship is nothing but a bunch of atoms formed in a special way. To these atoms we give the name "ship". So there is something but the existence of the ship is just created by our thoughts exactly like the meaning we claim to be objective/universal. But if you find a error in my logic please show it to me because it would really help.

You're right there we really don't now in what way we exist only THAT we exist. But in whatever world we live meaning would still (and only) be created by thinking beings.

My personal explanation to that question would be that meaning itself is created by thinking beings as I said. Therefore if those beings died out the meaning they created ceased with them. What I conclude from that is that there can be no universal meaning or purpose. Things just are and only humans (as an example for those beings) bring in the purpose/meaning. So there can be no objective/universal meaning or purpose or goal to follow. I also don't think nihilism must be depressing because if there were no such thing as objective meaning we would be totaly free in our actions and that I see as something good.