Reach Rank 2 with any Warframe.

After completing the tutorial, you'll have to choose between 3 different Warframes. Don't worry about picking the wrong one, since you can obtain and play any Warframe in the game itself after the tutorial (this may take a while though). While all 3 of them are great frames, Loki is more geared towards advanced players. Therefore I would suggest choosing either Mag or Excalibur.

Every time you play with your Warframe, you'll level it up, with a max rank of 30. Rank 2 won't take long at all.