Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
For all my PF players, what is your favorite gun? Personally, I enjoy the MP7 and the AK12 for their good all around performance with laser and grip attachments, and for secondary I usually use the REX revolver for when the enemy just won't die.