Always make it a priority to collect the Green Gems as you will need them if you want to restart the game.
To be able to level up you must complete objectives. Completing objectives will also earn you rewards and allow you access to new power-ups.
At the in-game application store of Temple Run 2 go to 'Get Free Stuff'. Once you have done that you will be able to follow Temple Run 2 on twitter which will give you 250 extra free coins to play with.
At the in-game application store of Temple Run 2 go to 'Get Free Stuff'. Once you have done that you will be able to 'like' the game on Facebook which will give you a totally free Gem.
The following can be obtained by completing the corresponding task.
Run 500 meters.
Collect 100 coins.
Score 25,000 points.
Score 50,000 points.
Collect 250 coins.
Collect no coins for 250 meters.
Collect the indicated amount of coins to unlock the corresponding character.
Collect 5,000 coins.
Collect 15,000 coins.
Collect 25,000 coins.
Activate the special powers and abilities like shields if you are struggling. Just double tap the screen to enable them.