Q: How much is citation one journal in another journal? How do I know this?

I am not sure I completely understand your question.

If you mean ‘how do you cite one journal (paper) in another journal,’ here are a couple of articles (based on the various style-guides) that you can refer to:

• APA Style cheat sheet: How to cite a journal article using APA Style
• How to cite an article: Quick tips on Vancouver and Harvard styles in citations

If you wish to learn about Q1 and Q2 journals, that is, journal rankings (and therefore, ‘how many papers published in a Q2 journal is equivalent to a paper published in a Q1 journal’), you may refer to this query by another site user: How to understand journal rankings by quartile?

If, finally, you simply mean ‘how important is citing one journal (paper) in another journal (paper),’ that of course depends on the relevance of the other paper to your paper. But yes, citing another relevant paper shows that you have done a thorough literature search in your area and are building on previous research, which is the beginning of good science and research. Hope that helps.