Depends on your reasons for getting into the hobby.
If you are in it because you want to go and play in tournaments competitively, go for the Necrons.
If you want to do it for the hobby/collecting side- just go with whatever army you like model wise or story wise.
As for just regular gaming- I would recommend any codex written by Matt Ward. With his books you don't have to look up what combinations work, you can generally just build whatever combo speaks to you personally and it has a good chance of doing okay on the tabletop.
(Unlike say Cruddances Imperial Guard and Tyranids, which only work in certain combinations)
Keep in mind that Necrons are very easy to paint if you just want to paint on a casual sort of level- Boltgun Metal, Black Wash, then work on the glowy green stuff, then you are done. But don't let that discourage you from doing something different and going all out on them- they can be painted to an amazing standard by some.
Tyranids used to be entirely a horde army, but these days you don't have to assembly line them- with their current book you can take genestealers or tyranid warriors as troops, and take lots of big monsters, meaning you only need a few models to run a tyranid army.
Heres a quick shortlist of armies-
Highly Competitive- Necrons, Grey Knights, Space Wolves, Imperial Guard (But only one build for the guard)
Horde- Swarm Tyranids, Infantry Guard, Cultist Chaos, Orks
Maneuverable Shooty- Dark Eldar, Eldar, Tau, Dark Angels
Gunline Shooty- Imperial Guard, Space Marines
Heavy Assault- Blood Angels, Chaos Marines, Dark Angels, Space Marines, Orks
Maneuverable but Squishy Assault- Dark Eldar, Eldar
Then there is Chaos Daemons... im not sure where they fit in right now because their new codex is so new I still don't have a clear impression of them.
The most important thing in my view would be to pick the army that you like the models of- no point picking necrons if you don't like painting endless amounts of robots, or picking Blood Angels if you hate the colour red.