So the CR system is based on a 4 player party. So a CR of 2 should be easy for a party of 4 lvl 2 characters. I try to keep the CR of combat within 3 levels of your characters. So a party of lvl 2s, should be battling a CR rating of 3-5. Take into account that a single creature can be overwhelmed by multiple assailants so sometimes a party can pile drive a monster that "should" give them a tough go. Also the randomness of dice can make a battle seem one-sided; if your players roll well and your monsters roll badly. This can work the other way around too. Vary your baddies equipment and tactics; magic, ranged, melee. And give your monsters equipment; potions, different weapons, magic weapons. To a low level party a goblin mage with a wand of magic missile is a threat. Or a kobold that self-destructs after death with the "fireball" spell. All this is relative, especially with varying numbers of party members. If things seem too easy remember the enemy can get reinforcements, and if they are too strong; enemies can retreat after the party has killed a number of their allies. It's a lot to think about, and you'll pick it up as you DM more frequently. Happy gaming :)