1. "Also" means that there are other points coming before the current sentence it is used it.
2. "Then" shows the direction or time-line of a procedure or order of the actions beind carried out. There must be an action occurring before the sentence in which "then" is used.
3. "And then" has the same use as "Then", However, conjunctions such as "and" and "Because" are seldom used in formal English. Schools normally frown upon such words being used at the start of the sentence. Novels, however, do use such conjunctions to start sentences.