We have been in that situation many times with friends. (Of course in the office my DW does it all the time, but that is a different question.)
We tell them to pick the itinerary first. WHERE they go is more important, IMAO, than what line they go on. For a first cruise, the Caribbean or Alaska would be our suggestions. Cruising the Med or Asia seem a bit of a big bite for a first cruise.
Then we say that all the main stream mass market lines are pretty similar and that they will probably be happy on almost any of them. The customary descriptions of Carnival as party ships and HAL as a staid ship for old folks are no longer true. So we suggest they choose by embarkation port and convenient dates. And after that we suggest they look at price.
The cruising experience overall is more important than the particular cruise line. However our own particular favorites are HAL and Princess, so all things being equal we would say opt for one of those.