To expand on what others have said, warframes give 200 exp a level and weapons give 100 exp. Due to how the exp system works in Warframe, the best method for gaining exp is killing enemies with your frames abilities, as 100% of that exp goes to the frame alone. Thus, leveling big AoE frames is inherently easier than leveling frames like Trinity. The best places to farm lots of mobs are defense and Mobile defense. For lower levels run Io and Callisto on Jupiter (~lvl22-25) or Kappa. For higher levels you can run Kiste (~lvl50) and Xini.