If you wish to leave no ambiguity about when the decision was made, you can change the word order
Yesterday, I decided to go there.
("Go" is indeed more natural than "come"). I infer that, because of the sequence of tenses used, both actions occurred yesterday.
I am sure there can be other contexts in which what I say may not be the law, but without a context this is the best I can do.
If, however, you say
I had decided to go there yesterday.
one cannot be sure that yesterday determines to go, but what is sure, is that the decision was made earlier yesterday, or on any other day before.
As for your second question about the sentence
I have been working in order to have good life conditions since I graduated.
I don't think it is incorrect, but it might work better like this
Since I graduated, I have been working in order to have good life conditions.
You may want to use in order to improve my life (conditions) instead.
I have been working since I graduated in order to improve my life conditions.