Just has many meanings, the main one is proper/morally right, eg He is a just person, because he follows the laws of his land. Only=solely/alone, but there are other meanings as well.eg She was the only one left in her village.

Also how I know just means simply.

Hi Paulo Jura has mentioned one meaning of 'just', but there is another which means the SAME as 'only'. for example: 'I have only two children' is the same as 'I have just two children'. In conversation, they are interchangeable. Also, you can use them together sometimes - 'he only just made it to the office on time', meaning that he arrived at the very last moment.