You can have short term goals, eg. Getting the biggest house or maxing out your skills, or getting more friends.
However long term you just play and make a family, then watch the children grow and them play them and make a family etc etc.
If you want a goal have a look at the challenges pages at simsresource.com or google Legacy Challenge.
Not sure where it was posted but my favourite challenge was to create a family of 1 adult and 7 toddlers. Without using cheats, hacked items or hiring any staff (no nanny, no maid) all you've got to do is get the kids to adulthood.