1. CP is important, but element types often matter even more. Always remember what's strong against what.
2. Here's an easy-to-read chart that you can save to your phone.
3. Just because a Pokémon is a certain type doesn't mean its moves will also be that type. So always check their moves!
The Geodude on the left has two rock-type moves: Rock Throw and Rock Slide. The Geodude on the right has Tackle, which is normal-type, and Dig, which is ground-type. So if you tried to use the Geodude on the right against a Pokémon weak to rock, neither of its attacks would do extra damage.