I would pick an army that suits your playing style.
Agressive - go for Chaos or Skaven,
Defensive - go for Empire or Dwarfs -
Elite - go for High Elves
How much time you have to paint and how good you are should also be a factor - if you are an avid painter and really good, you may wish to go for Brettonia. If you want a simple, quick paint job then perhaps something with lots of armour (Chaos?) or lots of uniformity (Tomb Kings?).
How much you want to spend is also a factor that you may need to take into account? Horde armies are more expensive. Some armies turn up more often on e bay that others and can be purchased, over time, at nearer to £5 a blister instead of the £8 if you order them direct from GamesWorkshop.
Just some food for thought....
Dave