Well it depends on how you'd like to play....
I think that as a start, probably pick Seasons up. It's one of TS4's best EP's and it ads more replayability to the BG and other EP's as well. And it makes playing with families more fun since you have the holidays and seasonal activities.
But it's really up to you, want to play celebrities? Go with Get Famous.
I think that the weakest EP, is Island Living, beautiful world but not a lot of gameplay, which in your case would especially not be the best thing to get, since you are just starting out.
I'd say go for an EP first, since they generally add the most gameplay.
But if you'd rather pick a GP, as a start, since it's cheaper, go with either Parenthood or Vampires for most replayabilty.