I always start with a single young adult, lifespan on long (and use a cake to age them up sooner if I want to). Fulfill their aspiration and build up some "family" money. I usually also have a story for them in my head- they like to party, so spend a lot of free time at clubs and bars or they are obsessed with their career or they want to flirt and be romantic with everyone they meet etc. When I've gotten them to a point where I have done whatever I want with them, sometimes they get married, sometimes not, but they always have a kid (or a few).
Then I pick the child I want to be my next main Sim and start a new story for them. When they are teens, sometimes I move them out on their own, sometimes my previous Sim dies. *shrug*
I always set out intending to play one family line for a long time, but I always end up getting bored with them and starting over by the fourth or fifth generation. Then I usually build for awhile before starting a new family line.