Which gods should I learn to play?

If you want to do jungle, I would suggest using Thanatos. He has no mana to worry about and you can easily sustain any trips through the jungle. If you want to support duo lane, I suggest you use Ymir. Many of the best people nowadays find Ymir's excessive CC to be a constant help to their ADC. but however if you want to ADC, I suggest using Cupid. His hearts can sustain you and your support, meanwhile giving you mana back if only your support takes the hearts. If you want to solo lane, I suggest Nu-Wa. She is one of the best lane-pushers in-game at this very moment (maybe besides ra). Mid-lane one of the best would be Ra, Isis, or Ao Kuang. Pfft you could probably use Nu Wa mid-lane too if you wanted, but beware the perils of your lane opponents, as people use different gods for solo and middle lanes.

As far as for the gods you are considering, I suggest Freya, she is a BROKEN god. She has extreme power out of virtually nothing. Not to mention her spell-vamp is very nice to have in any lane and will allow you some point of sustain. She is also a great lane pusher with her aoe nyan-blasts. I suggest you get her, she is fun in every game mode, and she is relatively easy to play. (I say relatively because, for some reason I have seen some people be bad at Freya, which is very hard to come by. Maybe they just bought her? Idk.) But Freya is simple and easy, smart and efficient.

Just depends on what appeals to you really.
While ra definetely is a staple, there's so many other gods that fit into little niches you might enjoy.
Want to play a high damage, high sustain character? Try thanatos!
Want an unkillable tank that can still kill? Hercules!
Want an annoying fish guy that has stupid wave clear? Poseidon!
The list goes on.
So yeah, just depends on what you want to play or like to play or are forced to play by your master.

Go for poseidon, he is a great pick especially in conquest as mid. He has awesome wave clear and very easy to gank lanes with kraken always ready to explode.