List of the most often used introductory words:
КСТАТИ – by the way
ПО КРАЙНЕЙ МЕРЕ – at least
ВООБЩЕ – generally
ВПРОЧЕМ – however, but
ПОЭТОМУ – therefore, thus, so
В ПРИНЦИПЕ – basically, as a matter of principle
ПО СУТИ – in fact
К СЧАСТЬЮ – fortunately
К СОЖАЛЕНИЮ – unfortunately
ОПРЕДЕЛЁННО – certainly, definitely
ДЕЙСТВИТЕЛЬНО – indeed
В САМОМ ДЕЛЕ – really, indeed
ПО-МОЕМУ – as I think
НАКОНЕЦ – finally, at last
КОНЕЧНО – of course, surely
К УДИВЛЕНИЮ – surprisingly
ВО-ПЕРВЫХ – first of all
ВО-ВТОРЫХ – secondly
НАПРИМЕР – for example
In writing, introductory words are always separated by comma.