So I more or less understand that it means "self", however I still don't get how to translate it. For example in the sentence:
Ganz ehrlich, ich wusste zu Beginn selbst nicht, ob ich die Beantwortung der Frage für wichtig halte.
I guess "self not" is not a proper translation of "selbst nicht".
So are there other usage of "selbst"?
You are making a small erroneous assumption here. "selbst" here is primarily connected to "ich" in this sentence.
"nicht" is connected to the verb ("wissen") here.
One could reformulate:
Ich selbst wusste nicht [...]
Then the translation gets clear:
ich selbst --> I self --> I myself
And then translating the whole sentence gives us:
Honestly in the beginning I myself didn't know, whether I believed the question's answering to be relevant.
It translates roughly to either "even" or "my-, your-, his-, one-, herself"/"our-, theirselves".
In all honesty, I myself didn't initially know whether I regarded the answer to the question as important.
In all honesty, I didn't even initially know whether I regarded the answer to the question as important.
You can also use "selbst" to emphasize that something was done by you
Das habe ich selbst gemacht! == I did that by myself!
Die Königin selbst könnte das nicht machen == The queen herself couldn't do that
