This is for personal finance but theres no one there
what type of interest is added to the principal in the account, so that the interest generates its own interest?

It is called compound interest - provided you leave the interest in the account.

Transition words are words like first, furthermore, besides, for instance, secondly, as a result using words like those should you write paragraphs showing time. you can start with first and then end with lastly or as a result